I sold my car via ebay a while ago and the chap wanted to pay me by bankers draft, so having discussed it with him and shown him the many orticles in the motoring press about fraudulent drafts we went to my bank to pay it in who checked there and then. As he was genuine he was fine with this.
Another option, if the buyer wants to pay by bankers draft is to suggest that he obtains one, with you, direct from a local branch after you've agreed the price. This way you know it's genuine as you saw it being written by the bank.
Cash is always a good option, and in my experience if a note has the metal strip through it, it's genuine... do check them all as if the buyer's genuine again they should not be nervous.
If you do accept a cheque let it clear first (and ring your bank to check it has), though this involves a risk for the buyer (as they don't know you either) so perhaps agree to write a contract or similar.
Never be afraid to ask.. if they've agreed to buy your car, unless they're dodgy, they'll understand your concerns.