The Federal Bureau of Alcohol, Tobacco, and Firearms (ATF) is responsible for enforcing federal firearm laws. Most US laws regulating firearm sales can be found in the Code of Federal Regulations, Part 478 - Commerce in Firearms and Ammunition. Conveniently, the ATF distilled those laws into Frequently Asked Questions, though buyers and sellers should remain mindful of state and local laws affecting gun ownership. The ATF provides information on many state laws and regulations.
California residents should be mindful of one law in particular: handguns must pass a 'drop test' to be legally brought into or sold in California. More information can be found on the CA DOJ Certification List page.