Do you have a copy of the release of liability form you should have submitted when you sold the car? I don't know what the DMV does with those forms because I know of 3 cases, one including myself, in which registration renewals were received for a car that was sold to a private party.

Do you have a bill of sale or copy of the transferred title signed and dated by the buyer? Try mailing in a copy with the citation.

And mail in another release of liability form - http://www.dmv.ca.gov/forms/reg/reg138.pdf This form isn't signed by the buyer so I don't know how it will hold in court.

Did the buyer pay by cashier's check or money order? Did you make a bank deposit that corresponds with the sale price?

Sixto
87 300D