At first the mythology was that any gun could be bought without NICS or the other official forms of checks at gun shows.
This was shown to be untrue but it was argued that gun shows were an easy venue for private sellers to meet. Some shows had tables of 'private' sales that were there constantly or folks who always had a a few guns they toted around for 'private' sales.
Thus, the show was seen and called in some places a risk for or attractive nuisance for potentially illegal purchases.
Private sales at a show would have to go through a check. I presume the parking lot would be patrolled.
That's the story.