Longer version: If you are buying from a private individual at a gun show, it's no different from buying a gun from a friend or neighbor.
If you are buying from a FFL (federal firearms license) holder at a gun show, they must perform all the steps required for a retain transaction.
At a typical gun show, the overwhelming majority of sales are by FFL holders -- as in about 90%. Only a handful are private individuals selling guns from their private collection.
The ATF is pretty stringent about what constitutes selling from a private collection, and buying/selling firearms as a business. If you are pretending to do the former, and are actually doing the latter, they will come down on you. And, they do "patrol" the gun shows looking for people like that.
If they require private sellers to go through FFL holders to sell a gun at a gun show, they'll simply drive those sellers away from gun shows and selling them elsewhere. The problem: a gun show is a secure place to sell your collection. You don't have to worry about meeting someone unknown to you to sell a gun, and you don't have to advertise to the world that you have a gun to sell.
Of course, the real goal is to track all private transfers. In some states, that's already happening. But, the reality is that very few firearms used in crime are acquired through legitimate means, either from a FFL holder or a private individual that legally owned the firearm.
Thanks all for your replies.
Let’s see if I have this straight.
If I meet another person attending a gun show and we strike up a conversation and make a deal that’s no different from my selling something to a friend or neighbor. Just a place where people who might be interested in such sales are likely to run into each other.
Here’s my followup stupid question. To have a booth at a gunshow, as opposed to just attending, does a person need a license? Or can I rent space and sell from my collection despite being unlicensed because I’m not “in the business?”