Of the 3 vehicles I have taken to CarMax, none was even remotely fairly priced ($2k+ below KBB trade-in valuation). The first one was so bad I laughed in the sales guys face and had to ask if he was serious. They obviously are doing something right but I would never even consider going to them to trade in/sell or to buy.
I guess I just got lucky then. They ended up giving me more than the KBB trade in value. Granted, I didn't check the private sale value. I just knew I didn't want to spend the time handling everything on my own. WILECYOT also got a great deal selling his car to them.