I've heard conflicting reports on whether Amazon co-mingles its own stuff with third party sellers, but some people have certainly claimed so. What co-mingling (it's opt-in for sellers) means is that Amazon doesn't distinguish which FBA seller gave them a particular item and will ship out the one that is cheapest to ship to that particular customer. Think of it as a giant distributed pile, where every seller's Koss Porta Pro end up. Now if I order one and the warehouse closest to me has units that a different seller handed to Amazon, I might get one of those, because co-mingling means that they're treated as identical. Remember that this is opt-in on the seller's part, and it's not entirely clear to me whether (or under which circumstances) Amazon co-mingles its own stock.