It's very simple: they're stored in different locations. If seller A is on shelf B, and seller C is on shelf D, then even though the individual items are identical, if you pick up a unit on shelf D you know it's from seller B.
They don't store identical products from different sellers next to each other.
I agree there's a counterfeit issue. Amazon agrees there's an issue, and actually has it as a risk factor in their financial reports. But commingling isn't a significant contributor to it.
You can check if the location it was supposed to be in is empty. From my understanding, the main issues happen during receiving; if it's received to the wrong shipment it'll be incorrectly marked in the system and unlikely to be fixed. Once it's received to the correct system everything else is fairly robust.
>If an item is pulled incorrectly, and then set on the return-to-stock pile, you have no idea where to put it.
If it's pulled, then there'd be a record of where it was pulled from, and that place would be empty.
Alternately, how does the picker know which item on the shelf to select to ensure they have selected the right seller's unit? Look at how amazon stores books in their warehouses:
https://www.google.com/search?q=amazon+warehouse+books&sourc...
Without a label on the book, how do they know which copy to pull from the shelf and put in the box?
I know Amazon says they track inventory, but prime shipped and sold-by-amazon.com boardgames arrive as counterfeits pretty regularly in the board game industry. You can find plenty of reports, with detailed pictures and manufacturer confirmation on BGG. There has to be something going on here, even if it may be more complicated than Amazon is commingling counterfeit and genuine articles.