A cars value is based on three things; 1) What the seller/owner thinks it's worth. 2) What a buyer is willing to pay. 3)...and least important of all, what NADA, KBB, or what "Black Book" says. Reason I say least important is because I have never seen NADA, KBB or the Black Book lay down cash to buy a vehicle. Google Black Book vs NADA. Black Book comes out each week and only goes to registered dealers.
As ToyoTech mentioned, the only real way to get an idea of the value of a vehicle is to research all the different selling outlets in your area, CL, Autotrader, Cars.com etc, to get an idea of what comparable vehicles are being offered for. You then have to decide what you think as a private seller each of these are actually going for.
In the end the only thing that matters is what the owner thinks it is worth and what a buyer would be willing to pay. If someone is willing to wait, a seller can usually find a buyer willing to pay what they are asking.
All that being said, I love seeing when someone posts about a vehicle being offered in the North East for $12,000 "Man that thing would sell for 20K here in TX." Really?!?!? Trust me if that were true people like me would be loading up for a little road trip all the time. While there is obviously a little difference in what a vehicle goes for based on geographic location it is usually not that much of a difference.
