If I have a dent on my rear bumper which would cost me 600 to fix,should I do it before going to carmax

I wouldn't fix it up! The biggest reason is that the Carmax can do the repairs at less than half the cost than you can. They are pros at evaluating cars...and can estimate pretty closely what it will cost for them to repair it. Depending on what the Carmax likes to keep on their lot, they may just wholesale the car out...in which case it makes even less sense for you to spend the money.

Probably not worth it unless it will greatly increase the car's value.

I agree with Joseph- no need to overspend on the car you're getting rid off.

I would look at the kelly Blue book trade in value between good and excellent. Evaluate if the difference between the 2 warrant the difference. Spending 600 to make 300 may not be worth it