if you sell more than 3 or 4 guns in a year, the BATFE considers you a "dealer". A dealer has to have a license, store all guns in a safe, be available for random inspections, keep records of all transfers, and perform background checks on every sale. It doesn't matter if they sell you a new or used firearm at a gunshow or the Walmart parking lot at 3AM on a Thursday - MUST run a background check and document sale.
If you sell less than 5 guns a year, you don't have to run a background check. But it's illegal to sell a firearm to someone who you know or have reason to suspect is a barred from purchasing firearms.
There is not, nor has there ever been, a 'gunshow loophole'.
(BTW - there's no hard limit on sales which makes you a 'dealer' to the BATFE, but if you're making money selling or fixing guns, you need a license or you risk jail)
The only exception is guns that are really old and are considered 'Curios and Relics'.
Buy/Sell/Trade without background check or records.