Amazon and a single person is a whole different situation.
Amazon is a multi-billion dollar company and people SE'ing won't even make a dent by looking at the amount of money they make.
As for a single person, that knife was probably their inventory. I won't even attempt to make any Derogatory comments towards OP, but a person with even a small moral compass would return that knife.
He sent me the trade offer, and even had to go on his mobile phone to accept the trade, which clearly states that he will NOT receive anything by this trade, hence the warning. If he is as careless as that, I don't really mind. The difference with this and Amazon etc. SE'ing is that he offered me a free thing because he was too careless to check something that takes half a second. SE'ing Amazon involves contacting them and tricking them into doing a refund by lying. So yes I get your point of you saying multi billion dollar company and a single person, but still.