Peter,
I am a big proponent black book auction prices. It will show what the dealer will expect if they sell/buy the car at auction. It allows you to know what the dealer thinks its true cost is. You can negotiate knowing that the dealer will get xxx dollars at auction.
Find a dealer that provided a black book appraisal on-line. It is a fantastic tool. I am also a fan of edmunds, and am able to get their appraisal without providing personal information.
Note the KBB used dealer pricing appraisals is an average of new car dealer average retail list price. It has nothing to do whatsoever of what a cars actual values is, or actual selling price. One should never ever use KBB retail price as a tool- it is a complete joke.
My goal is to never pay more for a car than it will sell for at auction. It takes a lot of time, work, and patience to find Sellers that wil do that. When a dealer sends a car to auction, its costs them in time, transportation, and auction fees.