I've sold two cars to CarMax - both good experiences. Each time, they gave me more than $1,000 over what I could get from a dealer trade-in. I've also had them appraise another car I was getting ready to trade in and they were $1,000 lower.
They rely heavily on KBB to sell their pricing to buyers and sellers. So, if you go in well informed, its easy to make a decision. They are definitely "no-hassle" from the buying and selling viewpoint.
When it comes to service though... Not too good. My daughter bought a '90 VW Cabriolet with a CarMax warranty. Of course, the car being a VW, just about everything electrical that could break did - locks, windows, radio, A/C, and on and on. To their credit, I think CarMax tried to fix the problems. But, they were just not capable of doing so. Their mechanics just don't have the factory training needed. So, they would keep the car inordinate amounts of time, and inevitably return it to my daughter with a lame excuse unfixed. She finally took it to a VW dealer to get the really annoying items fixed.
Lessons:
1. CarMax is a good no-hassle way to buy a late-model used car that is not likely to have problems. (If you are happy with the price.)
2. Don't buy any warranty from them - they simply can't deliver, even if the service guys are doing the best they can.
3. They probably will not be best price on any car you actually want to buy from them. But it will probably be a reasonably fair price per KBB.
4. I don't know how much they buy from auction (per the previous post) but I do know they get a lot of private parties selling them 2 to 3 year old cars instead of trading them in to a dealer. This seems to be CarMax's sweet spot.
5. If you like buying electronics from CircuitCity, you will like buying cars from CarMax. They are the same company, and have a unique "my-way" or no-way approach. They are trying to cut new ground in how used cars are bought and sold to the public. It only gets problematical when you venture outside that late-model car that they are set up to handle.
6. And, like CircuitCity, they really haven't figured out the service side yet. Someday they will though. (Well, maybe.)
Kevin T.