As said, gumtree for the cheapies, pistonheads followed by autotrader after that.

Never fail with these, but then i make sure the car is super clean inside and out, good clear photos in a nice area and a clear to the point write up with a competetive price.

If it wont shift, it is normally either over priced or the pictures/write up is naff and no effort put in so check both are spot on before sticking it up.