Autotrader is basically a mass listing of the specific model and year range you select for dealerships and used car places in the areas you select. They may have private sale listings but either the default setting doesn't give that or I have never/rarely had those come up. Keep in mind that the vehicles on Marketplace often have title issues and/or prior accident issues.
Dealers, including your new car dealers selling used cars, are also selling vehicles they bought at the auction with some (corrected) body and accident issues (no title probs). Sort of pisses me off to see the numbers but the point here is to look very hard at what you're buying.
Do you have an idea of what you're looking for? I'm guessing its either a fuel efficient smaller car, a truck, or something sporty -- car or SUV. At least, that's what I'd guess for a teenage boy.