There are various theories as to how the market determines the prices of used cars sold by private parties, especially relative to new cars. One theory suggests that new car dealers are able to put more effort into selling a car, and can therefore stimulate stronger demand. Another theory suggests that owners of problematic cars ("lemons") are more likely to want to sell their cars than owners of perfectly functioning vehicles. Therefore, someone buying a used car bears a higher risk of buying a lemon, and the market price tends to adjust downwards to reflect that. 
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