Engaged in the business....I kind of got lost in the legal stuff, so I'm going to use the main concept, doing it for money. Profit/loss isn't the defining point, like a silencer, it is the intent of doing it that is the crime, not whether or not it succeeds. Right?
Now, I know some people (and have been one of them in the past) that go to gunshows, maybe rent a table, and sell some of their stuff. OR trade it. We take the money made, if any, and get more stuff. I have come home from gun shows with more guns than I took. And with the same number, but different ones, and sometimes less than I took, and no cash (got fewer but more expensive guns). I'm not engaging in the business, I'm engaging in my hobby.
Now, maybe a month or a year or ten, some of the guns I got at the shows will go away (sold/traded) and maybe they'll even be sold for more than I paid when I bought them. Again, all I'm doing is recycling part of my collection. Some guns are, or ought to be catch and release....
I don't see that as engaging in the business...
Now, the guy that buys and sells and uses his profits to make the payments on his Ramcharger, him I say is in the business, because he is not making deals to get guns to play with, he's making deals to make money for his living expenses.
I knew another guy, a serious Contender guy, used to be at a lot of the shows, with a nice table, lots of barrels and stuff, some guns, frames, grips, etc. He got kind of shut down by the authorities, not exactly for being in "the business" without a license, because he wasn't doing it as livelyhood (or partial) so they didn't have a good case against him for that.
BUT, what he was doing was taking his stuff around to shows in a three state area, and selling handguns (his personal collection) face to face at shows. SO he kind of was violating the transfer laws, because he wasn't using an FFL and was "out of state". As I heard it, he didn't realize he was breaking any laws. Heard he got a stiff talking to about it (maybe paid a fine), and promised to behave. Haven't seen him recently, but the last time I did see him at a show, he just had barrels & accessories, no frames or complete guns.
It is really a matter of intrepretation. Have heard horror stories of zealous enforcers, pulling stings at shows, but I think those things are pretty rare, at least in my area. Still, I do have a rule, if I buy a gun at a show, I won't sell it at that show. Why should I? I bought it because I wanted it for some reason (type, cal, etc..) and I want to play with it before I decide I'm tired of it. Someone offering me more for it than I just paid makes me want to keep it even more! Someone offering a lot more than I just paid sets off my alarms.
One good reason we go to gun shows to sell, as private sellers is time. Say I decide I have half a dozen guns that are surplus to my needs and wants. I stand a fair chance of being able to sell them in a couple days at a gun show. It might take a couple months or more to sell them in the want ads, or on line.
Want to know why there are so many hobbyists out there "dealing" without licenses? Thank the Clintons. In their zeal to protect us all from the scourge of guns (and gun dealers) they raised the license fee. From $30 to $300!!!!
Net effect, twofold. Drop in numbers of licensed dealers (big drop) as a lot of guys who only did it as a sideline/hobby simply stopped, because of the high fee, AND an increase in the number of people who simply refuse to pay that kind of money for a small amount of guns bought/sold per year.