http://ag.ca.gov/firearms/pubfaqs.php
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:

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.

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))

Another good site for information is: http://www.calgunlaws.com/

It's none of my business but I'm curious. Why do you want to sell it?

move to a differnet state

Seriously, both of you go to a dealer and have him transfer it, it should not cost more than $25-50.