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Go online to a major auto retailer. Look for cars that are the same exact year, make, and model. Write down the price of each car with the same mileage as the one you're valuing. Repeat this process based on listings at local dealerships. Add the prices together and divide by the number of vehicles to get an average.
Sites like TrueCar (https://www.truecar.com/), Autotrader (https://www.autotrader.com/), and CarMax (https://www.carmax.com/) are all good places to start. These larger online retailers tend to carry a lot of inventory, so you’re more likely to find comparable vehicles in their search engines. You are going to search for identical vehicles with similar mileage numbers, so you need to find a lot of cars. This makes the bigger online retailers a great place to start. If you’re trying to find the value of a 2014 Toyota Camry, search “2014 Toyota Camry” into an online retailer’s search engine. Try your best to avoid deviating from the year, as the difference in price between a 2014 model and a 2015 model can be massive. As you continue to search across different platforms, write down the price of every car you find that is the same year, make, and model. Try to stick to cars with similar mileages and conditions to get a more accurate picture. If you encounter a listing that seems radically high or especially low, you can ignore it. There may be special modifications or unique damage to those particular vehicles.  Since you’re looking at cars that are the same make and model, the difference in mileage between listings shouldn’t be too massive. A margin for mileage should be around 20,000 miles in either direction. So if you want to find the value of a car with 100,000 miles on it, look for cars that have between 80,000-120,000 miles on them. Car dealerships have a little more freedom than major online retailers when it comes to how they price their cars. Since the price of a car is influenced by the supply and demand of the local market, find cars with the identical make and model at nearby dealers. Write their listed prices down on the same sheet of paper where you were jotting down the prices from online retailers.  You shouldn’t have to get up from your computer to do this. Almost every dealership lists their available inventory online. Wholesale auto auctions are not typical dealers, so you should avoid using these companies to source your information. Total up each price you wrote down, then divide by the number of prices. This will give you an average price based on all of the listings that you found. You can use this number as a reasonable baseline for the value of your car. Since you used a variety of sources, this evaluation will serve as a decent middle ground between the local and national market.