Congrats on your deal, my experiences with carmax differ.
The carmax in Indy was less than impressive to say the least. Granted this was 3 years ago, so it might have changed.
I went there looking for when i was car shopping. The cars I saw were significantly over book value. To add to my frustration, they had a '98 civic sitting on the lot ready to go to auction (if carmax doesn't sell a car on the lot, then they send it to auction where other used car salesmen buy it). The car had literally just been driven there a couple hours before I arrived.
Carmax wanted 10,000 for it, it was worth maybe 8000 at the time. I offered 7500 cash for it (around what the blue book was). I figured it was more than they would have got at auction.
At this point I learned that their no haggle pricing still applies to cars ready to go auction, and still applies to their overpriced cars.
I tried explaining my position to the sales lady, she was too dumb to understand what I was saying. My point to her was, if I wrecked the car a week after I bought it I would still owe a good 2-3k on it. It is one thing to be upside down on a new car, quite another on a 8 year old car. The sales woman just kept jabbering on about their stupid 150 point inspection program, like it was some sort of justification for charging 2k over book value.