NADA Car valuation is not an easy process. You need to take care of tonnes of things before you buy a car. And for the sellers, it can be a nightmare if the buyer has already done his part of research for the car's value. Always make everything transparent while dealing to avoid conflicts.
Find everything about NADA Guide - NADA Blue Book, Kelley Blue Book and the Blue Book Value of Used Cars. See how NADA Blue Book calculates NADA Car Value