There are plenty of people who can legally own a firearm that would much prefer to buy one used and not registered to them than one that is registered. I think this is one of the things that keeps used gun values so abnormally high.
THe current registration system is a feel good law I will let the ani-gunners have. Who knows, maybe it does stop crime. As it is set upo it would be nearly impossible for the ATF to systematically confiscate guns because many gun owners have no registered fire arms. I know of several people with no criminal record who go out of their way to bypass registration and have no registered guns.
Personally the guns I want are hardly ever available to me in FTF transactions. Those that are I buy that way.
I have never seen a table with more than one gun on it for private sale. I have seen several times where someone selling something besides "firearms" was selling a gun privately(say a reloading supply dealer). I don't think there is anything wrong with that even if they have that gun sitting out for 6 shows straight or they have 2-3 rifles.