Since most used car leads are going to be driven by your inventory, price, color, condition, description, etc....I think you will be hard pressed finding a pay per lead pre owned lead. That being said....the strength of cars.com and autotrader.com are by far going to give the majority of dealers the best options. Carsdirect.com is inexpensive for pre-owned but some dealers have a great amount of success with them. I would say that 50% of the dealers I signed up with carsdirect.com used program loved it. It's just a matter of trying things and seeing if you like it. I have found that perception plays a big part in the success of leads. If a dealer gets 2 or 3 deals in the first week or two than they love you but if not they don't. Give it a true 60 to 90 day trial and see how it works for you.