Funny you got donwoted the exact last time amazon inventory practices were under scrutiny here at hn there was plenty stories of people getting empty boxes and ipad cardboard cutouts

edit: dug out that discussion https://news.ycombinator.com/item?id=13924546

but there are plenty similar.

Are you sure it is not the other way around, that you have to opt out as a seller?

Anecdotally, a friend of mine runs a company that produces a unique fitness product, never opted into any kind of co-mingling as far as I know, as they are the only supplier in the world - and suddenly found themselves with comingled fake stock.

This is incorrect, kind of. There are essentially 3 ways to sell on Amazon. 1. Amazon can buy your product wholesale and they sell it. The manufacturer still has a lot of control on pricing and listing and etc. But Amazon has more control. In this case Amazon uses the UPC (or equivalent) to come up with the ASIN. In this case any 3rd party seller can list the same item with the UPC and it will show up on the same listing. They can even sell it via FBA where it is labeled as prime and stocked in Amazon's warehouses and if priced cheaper then will often get the buy box. So you could have 1000 positive reviews for your product and then all of a sudden a counterfeiter ships Amazon a bunch of what they say is the exact same product with the same UPC and it gets listed on the same listing. They can even make these stand out more by making variations on the product that then show up (like a 2-pack) that maybe you never sold.

Secondly there is Fulfilled By Amazon (FBA) this is when you sell the products and Amazon takes a commission and all the fulfillment fees to store and ship the product for you. In this case the seller has MUCH MUCH more control over every aspect of the listing. They can choose to have the ASIN generated by the UPC which can result in the same problem as above OR they can choose to have the ASIN generated by a number and barcode that is unique to the particular seller. This is called a FNSKU. Even if you know the FNSKU number which would be on a barcode on the product, ONLY the particular seller can create /ship product to Amazon using it. So Only the seller can variate listing or sell the product.

And thirdly you can do seller fulfillment which works list the last option in terms of listing, just with your own warehousing and fulfillment.

Yeah, but you can't tell as a purchaser.