No, the thief created the mess. All that follows flows from that. The current owner has recourse to the pawn shop, which is obligated to refund his money, since the transaction was illegal. If the pawn shop refuses, their customer should prevail in small claims court.

Getting distracted by the side issue of the serial number missis the point. Consider what law would apply to provably stolen items that don't have serial numbers, or were transferred to places where the serial number wouldn't be registered either way.

I understand that the current owner probably acted in good faith, but he wouldn't be the first person who was acting in good faith and still lost out. And as I said, he has recourse, so if the OP gives him any compensation, he'd actually be compensating the pawn shop, for whom this is simply the cost of doing business.