Since most of the sellers who have booth spaces at gun shows also have brick and mortar stores, they have Federal Firearms Licenses and the gun show circuit is just another avenue for them to sell their wares.
Since they are in the business of selling firearms, they are required to do background checks on all buyers.
https://www.dailywire.com/news/fact-checkers-rush-to-defend-downplay-joe-bidens-gun-show-loophole-claims