You guys need to understand both state and Federal laws apply in transfer. This guy is in CA. Unless you are trading a gun over 50 years old (curio/relic status), the transfer needs to be done by an FFL.
Federal firearms law...
Sales Between Individuals
An individual who does not possess a federal firearms license may not sell a firearm to a resident of another state without first transferring the firearm to a dealer in the purchaser's state. Firearms received by bequest or intestate succession are exempt from those sections of the law which forbid the transfer, sale, delivery or transportation of firearms into a state other than the transferor's state of residence.
Dealers
Persons who engage in the business of buying or selling firearms must be licensed by the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives of the U.S. Department of Justice. A special class of "licensed collectors" provides for the purchase and sale of firearms designated by the BATFE as "curios and relics." Class III dealers may sell fully-automatic firearms manufactured prior to May 19, 1986, and other federally registered firearms and devices restricted under Title II of the Gun Control Act, to individuals who obtain approval from the U.S. Secretary of the Treasury after payment of a tax and clearance following a criminal background check.
Violations of restrictions on Title II firearms and devices are punishable by a penalty of up to $10,000 and 10 years imprisonment.
And pertinent to this seller, since he is in California.
From the California Firearms Laws FAQ:
# I want to sell a gun to another person, i.e., a private party transfer. Am I required to conduct the transaction through a licensed California firearms dealer?
Yes. Firearm sales must be conducted through a fully licensed California firearms dealer. Failure to do so is a violation of California law. The buyer (and seller, in the event that the; buyer is denied), must meet the normal firearm purchase and delivery requirements. "Antique firearms," as defined in Section 921(a)(16) of Title 18 of the United States Code, and curio or relic rifles/shotguns, defined in Section 178.11 of Title 27 of the Code of Federal Regulations that are over 50 years old, are exempt from this requirement.
Firearms dealers are required to process private party transfers upon request. Firearms dealers may charge a fee not to exceed $10 per firearm for conducting a private party transfer. Example:
1. For a private party transfer involving one or more handguns, the total allowable fees, including the DROS, safety, and dealer transfer fees, are not to exceed $35.00 for the first handgun and $31.00 for each additional handgun involved in the same transaction.
2. For private party transfers involving one or more long guns, or a private party transfer involving one handgun, the total allowable fees, including the DROS, safety, and dealer transfer fees, are not to exceed $35.00. The dealer may charge an additional dealer-service fee of$10.00 per each additional firearm transferred.
(PC section 12072(d))