It varies from state to state, although there are several out there that have permit requirments, or FFL requirments, the majority do not have laws banning "Face to Face" private transfers.
IANAL, and YMMV, but in those states generaly speaking, you cannot knowingly sell to a felon, and by federal law you can't sell to an out-of-state resident.
The "Interstate commerse clause" of the Constitution is what gives the Federal Government the authority to force handgun sales and initial distribution to FFL's through the 1968 GCA in the first place.
Few people are brave enough to challenge it in court (IIRC, a guy just did recently, and won, in the 9th circuit court...) but it's not technicaly illegal from a Federal standpoint, at least, to manufacture, posess, and use your own home-made machine guns or silencers, as long as it's only posessed or used in your home state.
However, most states that do allow MG's or other NFA items, close this "loophole" by requiring all MG's, SBR's, SBS's, or suppressors be on the NFA registry, and the Federal tax paid anyway.
