[seanofjapan]

I'm going to be a contrarian and say that I think M should keep the cards.

This is basically a question of who should suck up the loss , A or M? (I'm assuming that getting the real person who should bear the loss, the brother, to pay up is impossible).

I think the over-riding principle should be that the person who was in the best situation to have prevented the debacle in the first place is the one who should bear the loss.

M was a purchaser in good faith who had no way of knowing (or reason to suspect) that the cards had been stolen. He is completely blameless and there was nothing he could reasonably have done to prevent this from happening.

A on the other hand was basically careless with his cards. I'm assuming a bit from the facts you have given that he left them with minimal to no supervision at his parent's house, while knowing that his drug addicted brother had access to them and might steal them. He was in a position to have prevented this from happening at all if he had just put them in a safe spot away from his brother, and was negligent in failing to do so.

While I can sympathize with A, I really see no reason why M should be forced to bear the cost of his loss. My opinion would be a lot different if M knew they were stolen or had reason to suspect they were stolen and turned a blind eye to that (ie bought them from an unreliable source, etc), in which case I would say he should absolutely return them.