http://definitions.uslegal.com/e/eng...ness-firearms/
I have been told as a rule of thumb before if you sell more than 3 guns in a year for profit you are in danger. The actual legal definitions are murky though as you can see above. Local laws may or may not apply too.
Btw dumbest deal is a SIG 716 FDE on gun broker for more than 5k right now.