Carmax is not all things to all people all the time. I have shopped, bought and sold with Carmax (pre Covid market) a few times. When I have shopped for used, Carmax was in the mix of all the other used dealerships when I shopped and compared.
The couple of times I have bought from Carmax was because there was very specific color and options of the used vehicles I was looking and only Carmax had it in their inventory. Not necessarily local inventory but had these vehicle shipped in from other regional Carmax dealers at no additional cost to me.
Another thing, having a high range 800+ credit scoring, I was able to finance with Carmax financial arm at such lower percentage rates that I could not even touch with other traditional used dealership lots even with my high credit score. So the delta above market I may have spent with Carmax shrank to the point most would not quibble over at any other retail used dealership.