The amount of effort and time required would just barely be covered by the profit. You also have to store it until sale. That's probably what keeps this from becoming a widespread scam.
I think you underestimate people's desire to scam the system. Buy $1000 mattress, leave in box, "return" mattress and get $1000 refund. Sell online (instead of donating) for $300-400. Rinse, repeat.
The "rinse" part is where it gets tricky. There are firms that track returns (at least for retail). I've seen people have their returns straight-up denied.
