[cloudsloth]

Edit: sorry for re-stating some of your points. I'm sort of delirious with a head cold right now.

What's pretty interesting is that it could be both.

Sellers get's a return. Upon receiving returned item they check that packaging is still sealed and when it is, they send it back to amazon's fulfillment center to be sold again.

Amazon receives the item. Amazon's system (obviously) doesn't open boxes. They at the very most check the barcode, visually inspect for sealed box, and check weight.

Now the items are co-mingled with other new items

Now a buyer can receive a box of clay when they buy from an entirely different seller.

Co-mingled items allow for a ton of fraud. They also save Amazon a lot of money on logistics. They make prime shipping possible. For Amazon, the acceptable rate of fraud is very high because everyone wants to be on their marketplace (huge sales volume) and the system they use allows them to offer such value at such a low cost.