Even when something by is sold by amazon, you could get something shipped to them by a third party seller. They keep all products with the same ID in one area, and pick whatever is on top.

If I started using FBA and supplying an item, it’ll go in the same box and chances are more likely an “sold by amazon.com” will sell my product, and I’ll sell an “amazon.com” product.

See the problem?

The argument wasn't that buying "sold by Amazon" would have a higher success rate, it was that returning would be more successfull than with some random pop-up account. This won't help against unnoticeable but still bad fakes of course.

"They keep all products with the same ID in one area"

No they don't. Google 'random stow' and you'll find articles about how Amazon stores incoming items. They are stowed somewhat randomly, but scanned as they are stowed. So the system knows where to find each unit again, and can use that to optimize picking paths.

By default, the system optimizes for speed and doesn't ensure that seller A's customers get the units sent by seller A and B's customers get those sent by B. This means that in a discussion about provenance of specific units, this is a distinction without a difference (or alternatively, all units of a product are in one area, which isn't necessarily physically contiguous in space).

Yes, my point is irrelevant to the main discussion about commingling.

I was commenting only because most people make the same false assumption (that all units of X in warehouse Y are stored together) and find the random stow model interesting when told about it.

> Because amazon doesn't add seller stickers/labels it's nearly impossible for them to even determine the seller if it's "Fulfilled by Amazon"

You are referring to mingled inventory which the seller has to opt into explicitly when they decide how to sell their product.

This is not the default, and in my experience, very few sellers choose this option. Most of the inventory is labeled with Amazon-proprietary bar codes that identify the seller.