Well, you don't need a complete registry. You could have sellers call a number and do a background check on a potential buyer. If the check goes through, a transaction number is issued. The seller gives the buyer the transaction number with the sale, and the buyer keeps it. If the sale does not go through, the seller must call and cancel the transaction number.

If challenged, the buyer provides the transaction number and serial number of the gun. The transaction number allows the buyer to own the gun after the date the transaction number was issued, as long as no new transaction number post-dates that number.

In practice, though, people are going to lose the transaction number. We can say, oh well, or we can go with a registry that has names, or we can ban private sales of guns.