When you trade in a car they put that amount towards the price of the car that you are buying also saves you from having the hassle of selling the car yourself.
When you trade-in you don't have to worry about selling the vehicle yourself or any of the associated costs (advertising, showing the car, etc). A dealer can offer a price you could not get yourself as an incentive to purchase a new vehicle. If the trade-in has any problems that could plague you down the road, selling the car to the dealer eliminates the bother.
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