OTOH, if that private individual rents tables at 6 different shows a year, buying and selling at each one, then a good argument could be made that he's "engaged in the business" and required to have an FFL.
I had an argument with a guy over that definition a couple of weeks ago that ended up with him barking at me and hanging up the phone. He sells guns on a very regular basis through various auction sites and forums. He's doing it repetitively and for a profit, and he's been doing it for several years.
There's certainly a tangible difference between him and a guy who sells a couple of guns he doesn't want anymore at the local show. The law is murky, and "repetitive" might be a gray area, but sometimes it's fairly obvious.