I think Spats is pretty much spot-on. If you're taking an occasional gun in trade for services, this is no different from your buying an occasional gun F2F for your own personal use.
If, OTOH, you're doing it regularly and then selling the guns to generate income, you would quickly fall into the definition of "doing business" in firearms.
I think the points made about not advertising your willingness to take guns in trade is a good one. Advertising would almost certainly be viewed as placing you in the "business."
While you are under no obligation to do so, you could have your local PD run the serial #s of guns you acquire through NCIC to see if they've been reported stolen. If you tell your customers you intend to do this, and they quickly retract their offer to trade, well, that tells you something right there!