This is the motor number or serial number required for identification purposes. Visit the California DMV's website and download a bill of sale.We've assembled a convenient checklist to help you get all your papers in order and sell your car legally and with a minimum of fuss. Complete the new owner information on the back of the certificate and sign it. Bill(s) of sale, if needed to establish a complete chain of ownership. We will be addressing how to fill out a bill of sale form for the sale of a car so let's get started with this guide to filling out a bill of sale form.