I have tried trading on two different vehicles at Carmax and have found them to be very very low in comparision to other dealers and especially selling outright. They are "low pressure" though which a lot of people like.
96 Honda Prelude...they offered 4700 I sold it on my own for 10,300
99 Ford Explorer...they offered a rediculous 3,570 and I sold it for 9,000
No way would I recommend them if it is out of your way but give it a shot if there is one near you...can't hurt just don't be surprised if it is way lower than you expect. I burst into laughter whent hey made the Explorer "offer" to me!