i would say carzone for the audience, they used to allow you advertise your car for as long as it took to sell in the past for a one off fee of 15 euro but now after 28 days they will charge you an additional 15 euro to renew your add.
i don't know if you regard that as value for money, i know a lot of people don't but carzone has got a good audience size.
by the way cars are not selling at the mo - privately or through dealers as everyone is waiting until after july to see how the market settles.