Vamps - thanks for coming back to us.
With regards to selling the car, a bank transfer has to be the way to go, if selling to a private individual. I'm assuming you are not selling an old banger for a few hundred quid, so anyone who wants to pay you thousands of pounds in cash, needs to be treated with extreme caution.
Without being alarmist, if you accept a large amount of cash, from an unknown individual, you could be breaking various money laundering laws.