>>Not exactly true. Any licensed gun dealer at a gun show has to do background checks. Only private citizens can sell guns without a check. Of course, private citizens can sell/buy guns anywhere without background checks.
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>Can some a private citizen buy lots of guns from licensed gun dealers before the show, pass the background check and get the guns and then sell them in the gun show for a hefty profit? Is there any limit on the amount of guns you can buy or sell (if you are not licensed)?

Sure they can. And all the gun's serial numbers will be registered to the citizen that can be traced back to them if they sell them to criminals.