I've received such sold-by-Amazon items as well, but I had assumed it was simple return fraud (customer returns wrong item in a packaging that looks unopened so

I've received such sold-by-Amazon items as well, but I had assumed it was simple return fraud (customer returns wrong item in a packaging that looks unopened so Amazon puts it back on stock). Of course I can't know for sure if it was that or comingling. Amazon definitely puts 3rd-party returns back in stock if it looks like still sellable as new - the seller help site says "We evaluate the condition of each returned item. If we determine that the item is sellable, we will return it to your inventory". So they probably do that for their own items as well.