Feel free to make firearm (federally, state law may be more restrictive) but only without the intent to sell. If you use it for a few years and then sell it, that's fine. If you "use" it for a day before selling it, that won't fly. It is also recommended to add a serial number to the receiver so LEO don't freak out. I believe only firearms manufactured with an intent to sell require serial numbers.
The whole hubbub over 3D printed guns is silly. People have already been legally and safely manufacturing firearms as a hobby for many, many years.
I thought it was the case that you could never sell it, only keep it for personal use. The entry at the ATF FAQ page (http://www.atf.gov/firearms/faq/general.html) is unclear, and I haven't actually gotten around to reading the statute.
The whole hubbub over 3D printed guns is silly. People have already been legally and safely manufacturing firearms as a hobby for many, many years.