We bought a 2001 Honda Accord in 2004 at CarMax for my DS to drive while at college. He graduated in 2006 and later that year I sold it to a co-worker's daughter who had recently totalled an Accord (DS bought his own new car). Last I heard a year or so ago, she was still driving it. Never had any problems with it.
My youngest DS worked at CarMax for a couple of years ~2009-2011 and he bought 3 different vehicles from them that they were going to send to auction because they didn't meet their requirements for selling at their dealership, but they had purchased them from individuals. Their policy is as long as you can get a car there, even if it's towed, they'll buy it. Cars that are 10 years old or older, have high mileage or prior major damage (turned bolts, welds where the frame has been "fixed"), they will not sell on their lot. But, instead they send them to auction. DS never had any mechanical problems with any of the cars he bought, but the first two were just transportation and were cheap. The third one was 10 years old, but in great shape and he drove it for a couple of years til he bought his first brand new car.
Point is, we like CarMax and have positive experiences with them. They are also great employers.