As a former Costco Employee, I'll clear it all up.
YES: Costco will take back ANY purchase, so long as it is intact, with all peripherals, manuals, bones (yes, they will take back food), wrapppings,
AND (the kicker)
Accompanied by the original receipt. Because receipts are marked leaving the building, they know that it was indeed a purchase. If you try bringing something back without a marked receipt, your membership is automatically revoked, and you are escorted from the building...🙂
Anyways, becasue Costco purchases directly form the manufacturer, they receive 100% value for the product, regardless fo condition, date etc. Sometimes they would open the RTV cage for us floor-sales guys, and we would basically take stuff piecemeal (ie drives, procs, cards etc). BUT, if soemthing looks entirely suspicious ie a 99% eaten cake, they MIGHT refuse it. (I've seen a steak returned that was nothing but fat and bone, the member said it tasted weird...🙂
So anyways, yes, they will take anything in perpetuity, as long as you have the receipt...