Signature of Seller. Gather Required Documents: Obtain the current title, a bill of sale, and identification.All title applications must include one of the government-issued photo IDs listed in Box 15. Detailed instructions for completing this form are located in the. A Texas motor vehicle bill of sale is a legal document used to demonstrate that a motor vehicle has been sold. County tax assessor-collector offices provide most vehicle title and registration services. Download. On the back of the title, you'll find a Title Assignment section. Fill this out with the buyer's information, your signature, and the date.