Using an Internet based car buying company seems to be a popular way of selling your car these days, so it is a good idea to have a comparison site for them. One of the most useful features is the customer review section which makes it easy to weed out the sites that over value and then always renege on the agreed price when it comes to closing the deal. Bear in mind though that, in my limited experience, all these companies will knock the price down to some extent. Best not to be too greedy, go with a company that has good reviews. You will probably end up with the best price that way..