The biggest issue with getting an accurate car trade in value is location of where you live and location of where you sell the car not only in the city or state but just the size and type of dealer, time of year, accurate condition assessment, mileage, colors, options, after market parts if you have any, market inventory, trade in manager, prior accidents, paint and body work.
The biggest factor to take into consideration is that almost no dealer actually uses NADA, KBB or alike, unless you are doing a trade in with a vehicle purchase and then they are blowing smoke at you. Meaning they are lowering the price of the car being purchased and giving you a higher trade in 95% of all dealers in the United States use Black Book which is normally not even available to the public. Black book uses auction sales, trade in market data and dealer sales to compile the database they have. Most used cars have reconditioning cost, depending on how old the car is and condition which can be $150 for cleaning or if tires, alignment and minor issue together can cost $1000-2000 more. Most used cars have a profit for a dealer of $1000-3000 depending on the age and condition some cars can give a dealer a lot more or even a negative amount if the car has hidden issues they did not discover during the inspection process.
The biggest impact on a car trade in value is if it has ever been in a car accident, we do diminished value claims and know that a car can easily loose thousands but many things are to be considered in a car that has been in a car accident, like what type of accident was it. If it is frame damage or uni-body it will loose the most value vs. a fender bender and/or a bumper replacement.
Think about how different states have different taxes, fuel cost, supply and demand of vehicles. If you live close to a port or to a manufacture of cars, usually the amount will be lower than in areas farther from the city. Type of car sells different in areas where a person would buy a truck for instance in the mountain area vs. a smart car in say California and Nevada regions.
In most instances, when you add after market rims and options to the car that could not be upgraded per manufacture options it reduces the value of the car and does not increase it.
So when you go to sell your car, take it to 3 to 5 places and you will see there is a difference in how much your offered.