Get the book value of your car by checking at car-pricing Web sites or ordering Consumer Reports Used Car Price Reports. Focus on the wholesale value and factor in the car's mileage and features. To see what buyers are asking for similar vehicles in your area, check car-buying Web sites or local classified ads. To gauge the true wholesale value of your car, take it to used-car dealers and ask what you can get in a straight-up sale. That will give you a benchmark for your trade-in and an idea of how much it can contribute to your down payment.
If you don't mind the extra hassle, you can get more money by selling the car yourself. But if you need the money for a down payment, you'll have to sell your old car before you buy a new one. You'll also probably pay less sales tax when trading in.