You're repeating a common misunderstanding about gun shows here which is that there are different rules for gun shows. There aren't. All licensed firearms sellers, i.e. all the people with booths, must do all the same background checks they would when selling from their stores. Meanwhile, all the non-gun-dealers (i.e. attendees) follow the same rules they would any other time of the year, which is that private-party transactions don't require background checks in some states. But that would be the same as if they sold a gun on Facebook or whatever. The dealers at the gun show are all doing the background checks as required by federal law (and additionally many states have more checks and/or waiting periods that also must be obeyed).