The solution is easy. Each gun has a title just like a car. If you sell it, you go to the Sheriffs office and transfer the title. That is a much better solution then banning them all together. Spread the word.
Probably be better to do it at a Gun Shop. I think that'd be more agreeable to the American public. And there doesn't have to be a title, don't give em any ideas, just a background check which is linked to the make and serial # of the weapon and date of transfer, as far as what data is retained with the FBI (no names or addresses). The gun shop keeps the paperwork on file, which does have all the other data, same as if they had made the sale, in exchange for which they charge a $20 fee. If that is not done, then it is a criminal offense to make the transfer.