Once co-mingling happened, they have no way of tracing one item back to one particular seller, unless the products have serial numbers. And Amazon is tracking those from Goods Receipt to shipping, which they are not. So if another seller is sending in counterfeits, with the same bar code, the whole co-mingled inventory is poisoned. And Amazon has no way of identifying the source of the counterfeits if those are not caught during goods receipt. No matter what they claim they can do.

It's not initial fulfillment that's the obvious problem, it's returns that are resold.

If I order 2 of something and they come from different sellers, and one is fake, how would they know which was which? Or that I selected the correct one to return?

Or if someone orders only 1, but recieve it and return a different one they bought from aliexpress.

Knowing that returned items get sold as new, it seems like the problem could compound, especially if the same item gets returned and resold multiple times. If it's a bad fake, that seems more probable then not.