I don't know about NADA, but it's my understanding that Kelly Blue book gets their info from DMV data. Vehicles bought from a manufacturer or dealer get reported accurately, but people engaged in private party sales often under report the amount of the sale in order to avoid taxes. (Horrible, I know
) So the values in the guide are inaccurate and always low.
I know someone who bought an antique car for $11,500 and reported to the DMV that he paid $1,500, cheating the tax man out of nearly $800. Since the car could have been a pile of rusty parts the DMV wouldn't know unless they investigated, which they very seldom do because of the expense.
Unless you are looking at a vehicle less than 4-years old, I would not trust the guide at all.