The car deakers use a little black book that is published monthly. I think it's just reffered to as the used car guided. If you go to an auction with dealers they are all using the same book.

They also use a program but I can't think of the name atm. It searchs for sales at auctions of like automobiles and what they sold for. If you need the name I can find out but it's another dealer only type of thing. Getting one of the books isn't hard either if it's like 8 years old it's the black one. If it is older then 8 years it's a grey book.