Wiki link: http://en.wikipedia.org/wiki/Gun_shows_in_the_United_States#Controversies
"Private sellers" and those who only make "occasional sales" - the types of folks that can rent a booth at a gun show three or four times a year and not have to legally register with the department of Alcohol Tobacco and Firearms - aren't required to conduct background checks at gun shows. So, a big store that comes down to a gun show will likely have the criminal background check stuff set up, require the forms, and whatnot. The dude with the rack of weapons that look like they were stolen from a Mexican drug cartel? Nope, he's exempt because he only occasionally sells guns.
Edited, Jan 3rd 2013 6:41pm by catwho