Did you ever try going to the dealer for a preliminary appraisal on your trade-in value before you waste your time?
You have to keep in mind that a dealer inspection is not going to be as stringent as a private buyer scrutinizing your car, ie. the guy is probably not going to be paying that much attention to every little detail and try to nickel and dime you. Besides, for a 9 year old car there's bound to be some expect wear and tear.
My point is, they might not ding you as bad as you think for some minor curb rashes
But if you are determined to paint it, I would advise that you don't make them black as it would probably would lower the value to most potential buyers. Paint it something closer to the factory color like silver or gunmetal.
Good luck on the process and congrats on your future 4runner purchase