Well the point of the whole thread is whether ATF cares. Believe me if they care about something you are doing...you will care pretty quickly as well.
I've had some interaction with the ATF.
The ones I've talked with seemed like pretty decent, regular people.
They are not going to care about a guy making a few bucks on a gun deal once or twice in his life.
There really isn't any way that you could even come to their attention for an occasional deal where you make $50 or so on a gun - at least not unless you all the sudden got a guilty conscience and sent in a signed confession detailing how you bought a used Ruger 10/22 last month and sold it for exactly $62 more than you paid for it. Even then, whoever read your confession would probably think to himself "What a waste of my time", then toss your confession in the trash.
Look at it from their point of view - it is to going to be worth their time to come and track down one guy who bought one gun and later sold it for few more bucks than he paid for it?
What they are going to care about is the guy who buys a dozen guns and then sells them to people who shouldn't have them from the trunk of his car. Eventually, something will catch up with that guy to get the ATF's attention.
But for you, me, or the OP, the ATF probably couldn't care less.
FWIW, I know this because I probably did manage to break a federal gun law once. I called the ATF and asked for advice as soon as I realized it. The guy I spoke to in the Pittsburgh field office told me "Yep, that was probably illegal. Don't worry about it though - just don't do it again."