Can a car be sold by just one of a co-owned car?

It depends on how the car was titled when it was purchased.

Most states give the option of titling the vehicle to the two buyers as either "and" or "or" owners. This would show on the title afterward.

As an example, if two people buy a vehicle with the "and" option, both of them must sign the documents to sell the vehicle.

If the "or" option is chosen, either owner can sell without the other owners okay.