So, in other words, they want no private sales at all. . . at least not among law abiding folks. Criminals will still steal guns or use straw purchases and sell guns to whoever has the cash or drugs to trade for them.
Legal gun owners who want to sell will just sell out of the trunks of their cars just like criminals do. Cash, and they can always say it was stolen. Ammo will be the big issue. And you can be sure the seller will know the buyer a long time.