You must refer to "trading guns" as "trading cash for guns"? You can't make a profit if no cash exchanges hands regardless of the other guns perceived worth. I have never heard of anyone getting an ATF visit, please share.
There have been a few guys on here who have had visits. The ATF has no rules about profit nor an amount before they decide you are dealing guns. They look at the combined picture of sales , length of ownership. If you bought a gun last week at the pawn shop and this week it is listed here and next week it turns up in a crime you are going to get a visit. They will usually give you the option of getting an FFL before arresting you. Go read about the 2014 bust at the Dothan flea market. The ATF used a felon as an informant/buyer so they get charged with selling to a convicted felon. There were also guys selling across state lines as well as felons selling. There are guys here who sold guns that ended up in New York crimes.