I have found that gumtree and preloved are good places to sell cars, FLe-bay is ok but be prepared to deal with some idiots, although i have never been messed about when selling a car, recently sold one on behalf of my father.
Autotrader is another option but it's quite expensive compared to others,
Or as other members have said become a gold member for £40 and sell away, not much more expensive than a good ad on Autotrader.