Carmax only sell cars substantially above their true market retail value. Trade in values offered are also below the normal trade in values well documented on line.
Did extensive research on true (full) retail values of used cars and found a much higher attempted billing (Sticker price), than that of other used car retailers. Salesperson knew almost nothing about the model I was interested in, even though it had sat on their lot for over 55 days. I had to teach the salesperson about features on the car. They will not negotiate even when you show them proof of retail overpricing.
Did not and will not ever buy from them as a result. Buyer beware and do your homework on retail values of used cars.