Trade in my car?

I want to trade in my corrola to a used newer US car.
1.Should i look for a car dealer? If i look for a dealer, it's easy to trade in my car to another one at the same time. If i buy a used one from a private seller and then sell my car, it will be very time consuming. But will dealers cheat me by hiding damages of a used car and dealers know how to change the milege of a used car?
2. There is a big black scratch on the front bumper of my car. Should i have it fixed before i trade in it in a dealer?
Thanks a lot.