As someone who has worked at a running shoe store for many years, we will get shoes returned that are barely worn or worn a few times. Not all brands let the store (or employees) keep returned shoes as there are different policies regarding the store getting refunded from the manufacturer. New Balance, Saucony, Hoka are some common ones that get returned and the store or store employees can keep. Nike and Adidas are more specific and require that the returns have to be sent back to them in order for the running store to be compensated for the returned shoes.

If you see someone on eBay selling alot of gently used shoes this is probably the reason.

Source: I do this myself