Car dealers are experts when it comes to deciding a trade-in price. They will use a number of resources, namely Kelley Blue Book and NADA trade-in value, but they mainly rely on their years of experience appraising cars. The main difference between Kelley Blue Book and NADA trade-in value is the data they use.
Kelley Blue Book gathers information to determine pricing by using information from car auctions throughout the country. It is very unlikely that a dealer will ever offer the full Kelley Blue Book trade-in value price since Kelley Blue Book prices tend to be a bit overinflated and dealers need to be able to make money on the vehicle when they resell it. There are, however, circumstances where a dealer would offer close to the KBB value. If the car was a very popular model or in short supply, a dealer would offer a better trade in price. Kelley Blue Book gives you a general idea of what you can expect for your vehicle on the private market and can be used as a good starting point for negotiations.
NADA, which is the National Automobile Dealers Association, uses actual retail sales data provided by its member car dealers—as there are sometimes large differences in the valuations of some vehicles. The industry consensus is that data from NADA guides tends to be more accurate. Car dealers also rely on The Black Book, which is an industry only guide not available to the public. They will often use the NADA guidebook as well as auction information to get a ballpark figure on pricing