Cash (in new 100s) or bank draft.
Whatever you do, do not accept a "money order", or "I'll pay you through ebay / paypal, that way, we are both protected". Both of these are high on the scam list.
Or, you could take a personal cheque from the guy and you keep the car until the cheque clears (takes about a week.) Once your bank confirms that the cheque has cleared, you give him the car. This works well, but takes a bit of time.
Beware if you advertise your car (as I've done several times) that the scammers come out of the woodwork. You will have all kinds of grifters calling you up looking to part you from your money. You'll hear about competing advertising services, guys who will sell your car for you for a fee (upfront of course), guys who can tell you how to get more for your car. They are ALL scammers and simply want your money.
Anyone who calls you up with anything other than buying your car - tell him to fuck off. Trust me on this.