As I understand it, there is no hard and fast rule for the number of guns which must be bought or sold by any one entity before that entity is "engaging in the business of" buying and selling firearms, such that an FFL is required. However, just because one buys and sells a gun or a few, does not mean that it is a business venture.
At some gun shows, I have seen guns labeled "private collection" on rented tables. My hunch is that it's just someone selling from, well, his "private collection." If a private individual wants to rent a table to do so, I see no problem. OTOH, if that private individual rents tables at 6 different shows a year, buying and selling at each one, then a good argument could be made that he's "engaged in the business" and required to have an FFL. Engaging in the business without an FFL is already illegal though, and won't be cured by universal background checks.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.