Only accept cash; don't take cheques, bankers drafts, building society cheques, any of that- make it CASH ONLY, and then count it.......
There is no time limit on a bank dishonouring a cheque: it can be cleared, and then a year down the line found to be dodgy, and the bank will want the money back which they paid out to you on the strength of that cheque......and don't forget the poor sod who sold his beemer on the strength of a bankers draft for 12000 which was initially cleared by the bank as genuine, then a week or so later identified as a fake: he lost the car and the money.......
Cash is the only reliable way to take money for a car. The other thing you need to do is satisfy yourself as to the ID of the buyer when you complete the V5.