Part of the thrill of going to a Gun Show is looking at everyone bringing stuff with them to sell. They carry pistols, rifles, knives, all sorts of things. Sometimes they will have a sign on a gun they are carrying with the price. Others have no signs. You simply walk up and ask "is that for sale" and go from there.
That is exactly what the guy in the ABC Show did. He walked up to someone in line that had a pistol in their hand and asked if it was for sale.
As long as you buy from an individual and not a licensed FFL Dealer, you can buy with absolutely no paperwork at all. A lot of guys that have extensive collections sometimes rent tables to sell their guns. You can buy from them without paperwork. Just because someone has a table does not mean that they are an FFL and paperwork is needed.
If you are not sure, just ask if the person is a FFL Dealer. If the answer is YES, then there will be paperwork and a NICS Check. If the answer is NO, pull out your cash and take the gun home with no other requirements.
AEA is correct. I sometimes have a table and sell from my personal collection. However, I always ask a few
questions: ie. Are you a legal resident of this state? Do you have any felony convictions or court orders that would prevent
you from legally buying a gun? I require a DL or CHL, and I copy that info into a notebook for my records. No NICS or other paper work required. The type of guns I am interested in don't normally interest the gangster crowd. Mostly, I run across straight up folks, good people looking for specific firearms for collections etc. As an individual, I reserve the right to not sell anything to anybody I don't want too. Side benefit of working for yourself!