I'll echo what others have said, it's really about the intent to make a profit. The ATF might make an inquiry if you're selling a lot of guns, but the main driver is profit. I know I've personally never made a profit from a firearm I've sold. At best I've broken even, and I try to keep it that way just to avoid the whole mess.
It's ok to incidentally make a profit, if that wasn't your intent when you bought the gun. e.g. a gun you own has really appreciated in value. You want / need some extra dough, and since you don't shoot that gun very often, you sell it for a large profit. In that situation, you are not a dealer. But if you bought it planning on making that profit, then you've run afoul of the law.