What are the rights of the cosigner to sell a car?

None, unless the cosigner is also on the title of the vehicle they have no legal rights to the property. When someone cosigns a loan for any reason they accept the responsibility of paying the debt if the primary borrower defaults. The only option a cosigner has in recovering money paid out in connection with the loan is to sue the primary borrower in the appropriate court, in the city or county where the borrower lives.