Careful.
According to federal law, if you're in the business of selling firearms, you must have an FFL. It doesn't matter whether you're selling FTF or transferring through a dealer.
For example, let's say that during the Obama gun buying frenzy last year, I went around New England buying up every stripped lower, upper, and complete AR I could get my grubby hands on. Let's say that I then marked them up to at least twice what I paid for them, and sold them here in the Classifieds forum to people with mommy/daddy issues and less than 300 guns. Let's also say that after four FTF transfers, I went through a dealer with everything.
The BATFE would come down on me like a ton of bricks because I was in it to make money.
Think about it... Most states don't have the 4-transfer-per-year limit. Why doesn't every Tom, Dick, and Harry open up a gun store without an FFL in those states?
With that said, it's probably way over-cautious to treat 4 sales/trades as a hard cap. Even if you're making a little dough, if you're enhancing your collection by buying more guns with the money you make, they'd be hard pressed to prove you're in the business of selling guns.
Thanks for that. I just learned something. While I have never sold a gun in my life I was under the impression that the ATF would only object to a high volume of personal sales (no FFL involved). I just assumed that one could sell unlimited numbers through a third-party FFL and be fine, regardless of motive or profit, and that going through the FFL somehow blessed the transaction. I had that wrong.