Earlier quoted context omitted.
There is not much to wonder about, Amazon does it because it critical to their fulfillment service, one of the key ways amazon makes money and keeps it 1-2 day delivery times is inventory mixing, i.e if you order from Seller X, you may actually get Stock that Seller Y shipped in. This is why they need to merge listings into 1. It is also why there is soo much fraud on Amazon, and I dont believe they will ever fix it,…
Do I understand correctly that seller X might get their reputation damaged if Seller Y shipped some counterfeit crap?
The way the service works, is that you the vendor will ship your products into a amazon warehouse, other vendors will ship the "same items" a amazon warehouse, all of these "same items" are mixed together in the inventory system, your account is has a credit of "X items" but not the specific items you shipped in to the warehouse
Example
Acme Vendor shipped to amazon 15 Logitech MX Mouses
Evil Vendor shipped Amazon 15 Counterfeit Logitech MX Mouses
When amazon gets the mouses it take all 30 and puts them in a big box, as the orders are filled even if you ordered from Acme, you make get one of the one Evil shipped into Amazon