Go to an auto auction business. Get yourself a "whosesale dealer's Blackbook." Do not get the consumer addition. Act like you know what you are doing when you go to buy it because some of the places will only sell them to dealers.
You may be able to get one from your bank as well.
These secret (not really) books are printed by-weekly and list the national average for the auction value of every vehicle in every year with every option ever made. This is the price that dealers use to buy autos at the auction and the price the banks use to determine how much they will loan you on a used vehicle if you do not buy from the dealer. There are different publications for cars and trucks.
This is where I always start.
I want to make sure neither of us are leaving anything on the table.