Car dealers are experts when it comes to deciding a trade in price. They will use a number of resources, but they mainly rely on their years of experience appraising cars. The dealer will give the vehicle a thorough inspection. They will often use the NADA guidebook as well as auction information to get a ballpark figure on pricing. Many dealers rely on the industry only Black Book as well. The biggest factor will be how much they feel they can get for the car when they resell it. A convertible in Alaska is not going to bring as much as a convertible in Miami. Another factor will be the dealers current inventory. If the dealership is overloaded with cars like yours, the trade-in value will be less. Finally a dealer will take into consideration what it will take to sell you a new car. They may offer a bit more on trade-in if they feel it will close the deal.
For more info, see Figuring Out Your Used Car Trade In Value