This always confused me. You have a bottle of glue sold by company X. Then you have 87 different people "buying" the glue in bulk, having it sent to Amazon, and selling it on Amazon as if it comes from their store:

Buying option 1: Company X glue from store A. Buying option 2: Company X glue from store B. Buying option 3: Company X glue from store C. ...etc.

But then Amazon says, "actually, these are all the exact same bottles of glue, so we'll thrown them all into the same bin, and no matter what "store" the people buy them from, we'll just grab them out and send them to the customer.

Now even without counterfeits, this is weird. What exactly is the point of store A, B, C, etc.? Company X sends the bottles to Amazon, they get put in one big pile, you buy them on Amazon, and Amazon takes them out of that one big pile and sends them to you.

The only thing purpose of the "stores" when you co-mingle inventory seems to be:

1. Plausible deniability for counterfeits. Hey, they told us they bought it from company X, we had no way of knowing they didn't.

2. Getting money from people trying to get rich quick in the marketplace. Some people will try all sorts of cuts to boost their Amazon sales in the hope that it will pay off later.

The reality is more complicated than you are assuming. A shockingly large number of vendors grossly mismanage their supply chains such that Company X can actually be legitimately undercut by reseller Company A on Amazon even though Company X produces the product! The mechanics of it are convoluted but legit, and there is a huge ecosystem of companies that arbitrage the legions of producers that are bad at managing their global supply chains.

Amazon has an interest in allowing these resellers of legitimate products to exist because it pushes down prices from the primary vendors, lowering prices for the customer. The primary vendors end up competing against themselves indirectly but they have no one to blame but themselves. This is the milieu in which counterfeit products exist.

If the producers of these products were consistently competent at managing their supply chains it would be much less of an issue because it would clear the field of resellers arbitraging the mismanagement, leaving only Company X and the counterfeiters which is a much easier problem to solve because you don’t have to worry about banning legitimate resellers. But that isn’t where we are.