They have always been very popular, but you will never find statistics because they are not recorded in a central database.
Guns, like most everything else, are personal property. Personal property changes hands all the time. I have bought several in private transactions; I daresay anyone with even a modest collection has acquired at least one using that method. Heck, I have acquired two very nice rifles at yard sales. And then there are all the guns that get bought and sold by people who see them as commodities, not a hobby.