A vehicle's Fair Market Value is generally the value a bank would be willing to finance the vehicle for if it were sold at a dealership. There are published guides (Black Book, etc.) that provide estimates of a vehicle's fair market value by using real data from recent vehicle auction sales. Because these guides use real data (what similar vehicles sold for at wholesale auctions) they are usually the most accurate and can differ from other online resources like Kelley Blue Book and NADA. Unfortunately, these guides are usually not available to the general public for free. Both Kelly Blue Book and NADA provide Trade-in values for vehicles which are usually (not always) close to the Fair Market Value of the vehicle. See the post titled "What is my Car really worth"
for more information.