As I understand it, assuming you're not a prohibited person, you can build non-NFA firearms for personal use in any quantity you want per Federal law - there may be local restrictions.
You can sell a personally built firearm if you eventually decide to.
You cannot build firearms for the purpose of selling them without a manufacturer's license, and all the paperwork that comes with it.
What may become touchy is if some Fed claims that you were building firearms for the purpose of selling them. I don't know what the case law is, but I suspect that in the case of a single sale or rare sales separated by a long stretch of time it would be difficult to prove you were operating as an unlicensed manufacturer.
If, on the other hand, you're frequently selling homebuilt guns - or even giving them away - you may get in hot water, since the feds may decide you're not really building them for personal use after all.