As long as he's selling to someone in the same state, the state regulations will determine what has to be done- Federally, you're just barred from selling to someone that's not allowed to own a firearm (a felon, someone convicted of domestic violence, addicted to drugs, illegal alien, etc)
If it's being sold to someone out of state (or a resident of another state), then in addition to the state regs playing a part, it must be shipped to an FFL, and the buyer fills out paperwork there.
If it's a pistol, and going out of state, you may have to bring it to an FFL to ship it to the out-of-state FFL (most FFL's won't accept a pistol shipped from an individual)
it's all spelled out here: in red, black & white:
ATF Online - Firearms - Frequently Asked Questions