Apparently they mix inventories from sellers, so if one seller is selling a fake you're probably going to get a fake.
Commingling your inventory allows Amazon to treat your inventory as being combined with the inventory that Amazon or other sellers have of the same kind and condition. When someone purchases an item from your inventory we won't physically distinguish your inventory from that of other sellers. This enables us to process customer orders more efficiently.
For example, if seller Joe Smith sells brand new iPods with UPC 8590905343 and seller Jane Doe sells new iPods with the same UPC, then when a customer places an order from Joe Smith for this iPod, we will fulfill the order with an iPod that is closest to the person processing the order in our fulfillment center. However, we decrement inventory availability for sale from Joe Smith, so he always has up-to-date knowledge of his stock.