Best bet is to use KBB private party value. When comparing this to the valuations we run at work, I find this to be pretty close to the true value of a car.

The only issue with KBB, NADA, or any valuation service is that it is not always what the enthusiast market is willing to bear. For example, 99.9% of the 240's sold in the Bay Area go to enthusiasts for hundreds to thousands of dollars over what the car is actually worth. Best bet is to try to base your price on similar age/condition vehicles for sale around the area.