As you said, its always good to be careful. Read through the laws and make sure you understand them to a point with which you are totally comfortable--if you still don't, ask a lawyer to further clarify. In reality, we are mostly guys with opinions here and you know what that is worth.....
Having said that, IMHO, in the "execptions" section of the above posted laws are the rules that make it possible for face to face transactions between two private citizens without FFLs. HOWEVER there are still rules, at least to my understanding, that need to be followed. Strawman purchases are illegal, even if some time elapses between the inital purchase and a private sale. If you are buying a weapon for someone else it is illegal and if you are buying the weapon with the sole intention of reselling it, that is also illegal. It is illegal to knowingly sell a firearm to a minor or to a convicted felon. And, it is my understanding, that the ATF will view the frequent buying and selling of firearms at a profit as an activity needing an ffl--even if you might consider those sales "private face to face" transactions.
Good luck in your sale....