A great place to start off looking for used cars is Buy Your Car. This is a company that pride themselves on offering the cheapest prices for second hand cars from private individuals, used dealerships and independent dealerships. They have a super-simple search engine: just enter your preferred car brand and model, any other car features you desire and your price range and away you go! One of the best things about this site is that it is a quick and simple way to find the used car that you want, without having to trowel through thousands of advertisements.
Another excellent resource is Auto Web. They have a basic search engine where you simply enter the make of the car, and your minimum and maximum budget. They also have a more advanced version, in which you also enter your preferred transmission type, body style and type of fuel used. If you would also like to advertise your used car, this site offers a free advertising service! You can show up to eight pictures of your car, for thousands of perspective buyers to see!