I think this depends on the state and the gun show promoters rules. I am pretty sure here in WA at Washington Arms Collectors shows, non-FFL holders (private citizens) can rent a table and sell their collection, but only to members (who have been checked).
I think there are laws regarding how many sales in a year makes you a dealer and you would need to get an FFL....
Does this not constitute a loophole?
I think we can argue about whether it is a significant problem, but not that it provides a legal way for people to sell guns in a public venue without being registered or licensed, etc.
Would this also apply at, say, a flea market? Could I sell a gun there without paperwork other than what might be required by state law?