Still one of the best ways to work out the value of your car is to scan the classifieds and seek out cars of the same make, model and a similar age, mileage and condition to yours.
This will give you a very accurate idea of what sellers consider fair and what buyers will pay.
If you want a quick sale, pitching your car just under the going rate will help, but going in dramatically lower than the going rate will leave buyers wondering what is wrong with the car that it's so cheap.