Motor vehicle ownership is evidenced by a title certificate. This is a document created by the government agency such as a DMV or secretary of state proving the ownership name and VIN# for the asset. It is an official legal document of ownership.
A bill of sale is only a receipt of money and statement of an event. It does not guarantee ownership and is insufficient for a transfer of title by the DMV. It may be a necessary record in order to apply for a title, but by itself a bill of sale for a vehicle only is only an agreement created between two private parties to exchange money for a vehicle on a specific date.
The only way to ensure a valid ownership for a vehicle purchase is to receive a properly signed and verified legal title document.