Why should he be nailed for anything.
He FOUND it.
No, actually he didn't find it... he bought it from someone who found it ... you see... :-)
There was no legal, perhaps an ethical, obligation to return it.
Ummmm.., no..., there was a legal requirement for him to return it, when asked to return it. And he did. That wasn't the problem, though.
What crime was committed?
It wasn't legal for him to buy that property... :-)
Explain this. If I found an object in the street, couldn't I sell it on Ebay?