[malandrew]

They could comingle and still solve the problems whenever they get stock from a seller they slap on an Amazon barcode that identifies both the UPC and the Amazon seller. That information gets included in the purchase details and when problems are identified with a product they pull those products and identify people who got the bad product. To cover the cost of that problem they can require sellers to insure their product or put down a deposit to cover the cost of a recall.

[jon-wood]

They effectively do require a deposit in case of returns in the way they pay. When you ship a product sold on Amazon they take the money immediately, but they don't pass on your cut until a few weeks later, allowing them to issue refunds if neccesary without losing anything themselves.