gumtree? Don't let anyone test drive, allow RAC or AA inspection at your house at their expense, cash sale or clear the cheque before they take it away.
Ensure that on the receipt the wording " sold as seen" is stated.that way there is much less of a comeback should the car have a problem soon after it is sold.I personally would use Autotrader.
Yes always put sold as seen on receipt and full cleared payments before car goes. and also ensure log book/registration book (V5 i think) is properly completed dated and signed, and that the right people have the right bits of this to post off to DVLA, this is very important as you can be fined if its not done right (so we were told when we bought car at garage) . auto trader, ads in shop windows locally, local newspapers and word of mouth seem to be good way to sell
Be honest, if not it will backfire on you, if there is a fault tell them, get a price in your head do your home work from the likes of Auto Trader or EBay, if on a test drive make sure the buyer has Insurance before you set off, no Insurance No test, Simple & go with them, if they are with a car take their Reg No, drop a few adverts in your local shops / Asda / Tesco.