Knowing how much inventory a competitor has on Amazon tells you a lot: how confident they are in a product, whether they're about to stock out, and roughly how fast they're selling.

Here's how to find it.
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The 999 Cart Trick

The oldest trick in the book, and it still works.
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1. Go to the competitor's product listing.
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2. Add the item to your cart.
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3. In the cart, change the quantity to 999.
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Amazon won't let you order more than the seller has in stock. If they have 447 units, the cart will tell you: "This seller has only 447 of these available." That's their live inventory count for that offer.
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Where the Trick Fails

If the seller has set a maximum order quantity limit on the listing, the cart caps you at that limit instead of revealing the true stock level. If you type 999 and the cart quietly adjusts to 30 with no availability message, you're looking at an order limit, not their inventory. Many experienced sellers set these limits specifically to block this trick.
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Tracking Inventory Over Time = Estimated Sales

One inventory snapshot is interesting. A series of them estimates sales.

If a competitor shows 500 units on Monday and 430 on Friday, they moved roughly 70 units that week. Check daily, and you can build a decent picture of their velocity.

Two things make this shaky:
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• Replenishments. If they restock mid-week, your math breaks. Inventory going from 430 back up to 800 doesn't mean negative sales,  it means a new shipment hit FBA.
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• Quantity limits. If the cart trick is capped, you never see the real number, so you can't track the real change.
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Treat time-series inventory tracking as directional, not gospel.
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Tools That Do This for You

Manually running the cart trick every day doesn't scale. A few tools automate it:
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Keepa

‍The popular Chrome extension tracks stock levels on individual listings over time, so you can see inventory history charted alongside price and sales rank. Great for monitoring a handful of ASINs.
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SmartScout.

This is where it goes from a per-listing trick to actual market intelligence. SmartScout shows inventory at the product level, but also rolls it up across an entire brand or an entire seller. Instead of checking one ASIN at a time, you can see how much stock a competing brand is holding across their whole catalog, or how deep a specific seller's position is.
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The Brand Use Case: Watching Your Own Resellers

This isn't just for competitor spying. Brands use inventory visibility on their own listings to monitor resellers.

If you're a brand fighting unauthorized sellers, the enforcement move depends on what you're up against:
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• A seller holding 12 units is probably a retail arbitrage flipper. They'll churn out on their own. A cease-and-desist letter is likely overkill.
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• A seller holding 2,000 units has a supply chain. Someone upstream is leaking product. It could be a distributor, a retail partner, or a gray-market source. That's worth a serious investigation and real enforcement.
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Inventory depth tells you whether you're dealing with a nuisance or a supply chain problem, and that should shape your response.
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SmartScout's Seller View on Brands and ASINs

SmartScout maps out every seller on a brand and on each individual ASIN. For any brand, you can see who's selling it, how many listings each seller is on, their share of the Buy Box, and how much inventory they're holding.

At the ASIN level, you get the full seller list on a single product. This is useful for spotting which listings are attracting the most unauthorized activity and which sellers keep showing up across your catalog.

For brands, this turns reseller enforcement from whack-a-mole into a prioritized list: biggest inventory positions and most persistent sellers first. For competitors and prospectors, it's a fast read on how contested a brand's distribution really is.
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Bottom Line
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• The 999 cart trick reveals live inventory, unless there's a quantity limit.
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• Track it over time for rough sales estimates, but watch for restocks.
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• Keepa handles single listings. SmartScout handles products, brands, and sellers at scale.
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• For brands, inventory depth is the single best signal for deciding which resellers deserve enforcement.

Your competitors are bidding on your keywords right now. Amazon doesn't have a public ad library like Google or Meta, so seeing what they're doing takes the right mix of tools. Here's the full toolkit, organized by ad type.
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Know what you're tracking

Amazon advertising comes in three main flavors:
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• Sponsored Products — individual listings in search results and on product pages. This is the biggest ad type by far, typically 60–80% of all Amazon ad spend.
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• Sponsored Brands — the logo-and-headline banners at top of search. Second-largest share of spend.
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• Display and DSP — programmatic ads on and off Amazon. Smaller share for most sellers, but important for big brands running full-funnel.
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Since Sponsored Products and Sponsored Brands make up the overwhelming majority of ad dollars, that's where competitive intelligence pays off most.
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Sponsored Products and Sponsored Brands: use ad intelligence tools

Sponsored placements are publicly visible on search results pages, which means tools can crawl keywords at scale and reconstruct who's advertising where.

SmartScout's Ad Spy does exactly this: look up any product or brand and see which competitors hold sponsored placements, on which keywords, and how consistently. No manual searching required.

This is helpful identifying advertising gaps. You can trust Amazon’s automatic targeting. But often it can miss something that your competitor has found.

For the trend line, SmartScout's share of voice tracks historic ad presence and estimated share of spend across one or many keywords at a time. That history matters: a competitor holding 40% share of voice on your hero keyword means something very different if it was 15% three months ago.

Whatever tool you use, it should answer: Who's consistently on my top keywords? Is anyone conquesting my branded terms? Whose share of spend is rising or falling? Which keywords are competitors buying that I've never considered?
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Display and DSP: use ad intelligence platforms like Sensor Tower

Display ads aren't tied to visible keyword auctions, so they need a different class of tool. Platforms like Sensor Tower (which absorbed Pathmatics) track display creatives, placements, and estimated spend across the web — good for seeing a competitor's creative strategy and directional spend. You won't get keyword-level precision here; for Amazon-centric brands, this is a complement, not the core.
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What Amazon's own console gives you

Amazon won't show you competitor campaigns, but it will show you the shape of the competition:
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• Impression share and lost impression share (including top-of-search) in Sponsored Products and Sponsored Brands reporting. If your impression share is 22%, competitors own the other 78%.
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• Brand Impression Share report under Insights and Planning, with metrics like Lost Top-of-Search Sales.
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• Brand Analytics / Search Query Performance — your share of impressions, clicks, and purchases on top queries.
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The catch: Amazon tells you you're losing share, but not to whom. That's the gap third-party tools fill.
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Manual methods (and why they don't scale)

Searching your keywords in an incognito window and browsing competitor product pages is worth doing — it's the best way to see actual creatives and messaging. But as a tracking system, it falls apart fast: results are personalized and change constantly, you can only sample a few keywords at a few moments, and you can't estimate spend or trends. Manual research tells you what one shopper saw once.
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A few things worth knowing before you start

Share of voice is a leading indicator. Visibility shifts usually precede sales shifts. A competitor ramping sponsored share on your keyword set is often a preview of BSR movement a few weeks out.
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High spend isn't smart spend. A competitor ranking well organically while spending less than you is a listing-and-reviews signal, not an ads signal. A competitor buying huge sponsored share with weak organic rank is burning cash and that can be an opening. 
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Defend your brand. Conquesting is standard practice. Check who shows up on your branded searches before deciding you don't need a defense budget.
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Don't copy blindly. A competitor's keyword list reflects their margins and catalog, not yours. The goal is finding gaps — keywords they overlooked, terms they overpay for, segments thin enough to win affordably.
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Bottom line

Sponsored Products and Sponsored Brands are where the money is, and they play out in public — the right tools can show you who's bidding, where, and how spend is shifting over time. Add display intelligence for the programmatic layer and your own console data for ground truth, and you stop guessing at competitor strategy and start reading it.

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The protein bar was built for this moment: concentrated, shelf-stable, portion-controlled. It's the one format losing ground. Amazon's data explains the paradox.

‍GLP-1 medications created the largest protein demand shock in a generation, and they did it by making people eat less.

Over 9 million Americans are now on Ozempic, Wegovy, Mounjaro, or a competitor. Clinical research shows that 25 to 40 percent of the weight lost on these drugs comes from lean mass rather than fat. A study presented at the Endocrine Society's annual meeting put the figure at roughly 40 percent for semaglutide users. The clinical response is now standard: 1.2 to 1.6 grams of protein per kilogram of body weight daily, roughly 90 to 120 grams for most adults, plus resistance training, to protect muscle during treatment.

At the same time, appetite falls by 30 to 50 percent.

That collision is the entire story. Millions of people were handed a protein target higher than they've ever hit, at the exact moment their capacity to eat collapsed. Every one of them became a shopper with an unusual and very specific set of constraints. SmartScout data shows Amazon's protein shelf reorganizing around those constraints in real time.
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The Muscle Defense Signal Is The Clearest In The Dataset

The first thing a GLP-1 user hears is that they will lose muscle unless they intervene. The commerce response to that warning is already massive, and the biggest signal isn't protein.
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Creatine generates $720.8M in annual revenue on Amazon, growing 90 percent year over year. The search term "creatine" draws 620,729 monthly searches and is up in every single tracking window SmartScout measures. Nothing else in this analysis grows in all four. Creatine is the most evidence-backed muscle preservation supplement available, and it's now standard in GLP-1 lean mass protocols.

Protein powder tracks alongside it at 603,335 monthly searches, up 59,279 year over year.

This is the purest GLP-1 signal on Amazon: not weight loss products, but muscle defense products.
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Liquid Is Winning The Smaller Stomach

A 30 to 50 percent appetite reduction turns volume into the binding constraint. The question stops being "how many calories" and becomes "how much protein fits before I physically cannot continue."
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Liquid wins that math, and the data is unambiguous.
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*Parent Nutrition Bars category; Protein Bars subcategory is flat within it. Source: SmartScout

The search data tells the same story, only sharper:

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Same nutrient. Same consumer. Opposite direction. Liquid gained roughly 57,000 searches over twelve months. Solid lost 2,600.

Ready-to-drink protein is the format that answers the volume constraint. Thirty grams, no chewing, no satiety penalty, tolerable when nauseated. Core Power and Premier Protein now split 55 percent of that category, and each spends around 15 percent of its ad dollars defending the position. That's why "protein drinks" costs $9.72 per click while cottage cheese costs $0.70.
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The Protein Bar's Real Problem Isn't The Format. It's The Ingredients.

This is the constraint the industry has not priced in, and it's where the protein bar breaks.

GLP-1 drugs work partly by slowing gastric emptying. The documented consequences are nausea, constipation, bloating, and reflux. Everything downstream slows with it.

Sugar alcohols are the standard tool for making a high-protein bar taste sweet without sugar. They're also a well-documented cause of bloating, gas, and GI distress in people with normal digestion. For someone whose stomach is already emptying at half speed, they're not a minor tradeoff. They're a trigger.

The brand leaderboard reflects exactly that:
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Source: SmartScout, U.S. Amazon marketplace, August 2026

Of the top 12 protein bar brands by revenue, 10 lost revenue month over month.

‍And the one brand taking meaningful share is doing something that should not work on paper.
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The Brand Winning The Protein Bar Shelf Has 40% Less Protein

TRUBAR is gaining share with 12 grams of protein per bar against Quest's roughly 20. Forty percent less of the nutrient the entire category competes on.

What TRUBAR's product titles instead repeatedly lead with is "No Sugar Alcohols," alongside 12 grams of fiber. Its top listings rate ranges from 4.3 to 4.8, compared with a category average of 4.22.

A brand is taking share by offering less protein and better tolerability. That only makes sense if the marginal buyer's binding constraint is their gut, not their macros. That is a GLP-1 buyer.

Fiber tells a parallel story: "fiber supplement" draws 158,468 monthly searches and jumped 28,689 in the most recent month, consistent with a population managing constipation as a treatment side effect.
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Real Food Is Beating Engineered Food, And The Ad Auctions Are Still Empty

When someone eats three times a day instead of six, each occasion carries more weight. In the search data, GLP-1 users are gravitating toward things that read as food rather than supplementation.
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Source: SmartScout

Those cottage cheese and Greek yogurt numbers deserve a second look. Each added roughly 100,000 searches in a single month. Each has just 9 advertisers. Each costs under $1.35 per click, while protein drinks cost $9.72. Cold chain logistics are harder, which is exactly why the auction is still empty.

Good Culture turned cottage cheese from a 1950s diet food into a protein brand and now holds 33.7 percent of the Amazon category with just seven products. Chomps overtook Jack Link's in jerky, $3.46M against $3.30M monthly, using 59 products against Jack Link's 215. Both are real food, clean label, and built for a shopper reading a macro target rather than browsing a snack aisle.
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Inside The Bar Aisle, The Growth Is Everywhere Except The Protein Bar Shelf

Put those constraints together, and the protein bar should be the defining product of the GLP-1 era. Concentrated protein, shelf-stable, no preparation, portion controlled. Instead, the growth inside Amazon's bar aisle is routing around the main shelf entirely.
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Source: SmartScout

Protein Bars is the largest subcategory by a wide margin, the most crowded (364 brands, 2,882 products, averaging 612 sellers per listing), and the only one not moving. The sparse formats are compounding at double and triple-digit rates. The dominant one is standing still.

The GLP-1 demand wave did not skip the bar aisle. It routed around the main shelf.
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The Brand That Understood: Protein Donuts Outselling Protein Bars

Legendary Foods is the largest share gainer in Food Bars, adding 6.3 points of share and roughly $194,000 in monthly revenue in a single month. It does not sell bars. It sells high-protein cinnamon pastries at $186,000 per month, chocolate pastries at $154,000, strawberry pastries at $130,000, cinnamon rolls at $100,000, and protein donuts that rank number one in Amazon's Donuts subcategory outright.

Its ratings sit between 3.9 and 4.2, below the category average. It's not winning on execution. It's winning because to someone eating three small meals a day with reduced appetite and shifting taste preferences, a protein donut is food and a protein bar is a supplement. Same macros, entirely different proposition.
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The Tell: Nobody Is Searching For The Drug

The Amazon search term "glp 1" draws 27,725 monthly searches and is down roughly 4,900 year over year.

That may be the most useful number in this entire analysis. The GLP-1 commerce wave is not visible in GLP-1 searches. Shoppers aren't typing the medication into Amazon. They're typing creatine, protein shake, cottage cheese, and Greek yogurt. The brands capturing them are the ones that solved for the physiology without ever naming the cause.
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What Smartscout's Data Adds Up To

The signals across the dataset point in one direction:
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• Build for the constraint, not the macro. The GLP-1 shopper isn't short on convenience. They're short on appetite, volume, and gut tolerance. Every category winning right now solves one of those three. Every category losing is optimized for grams per dollar.
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• Tolerability is wide-open positioning. TRUBAR is gaining share with 40 percent less protein and a "no sugar alcohols" claim. No major incumbent is competing on gastrointestinal comfort for a population of millions who now need it most.
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• Format is beating formulation. Legendary Foods grew by putting protein into pastries and donuts. Inside the bar family, the fastest growing subcategories are the ones not called protein bars.
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• The cheap traffic is refrigerated. Cottage cheese and Greek yogurt each draw roughly 300,000 monthly searches with under 10 advertisers and CPCs below $1.35. Protein drinks cost $9.72. Cold chain is harder. That's why the opportunity is still sitting there.
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• Never name the drug. Searches for "glp 1" on Amazon are falling. The brands winning are answering the symptom.
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Protein has never sold better on Amazon. The protein bar missed the wave, because it was engineered for an appetite that 9 million of its best prospective customers no longer have.

Data: SmartScout, U.S. Amazon marketplace, trailing 12 months through August 2026. Clinical context: Endocrine Society, published GLP-1 body composition and side effect research.

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