I bought my Mustang from Carmax. I actually did enjoy working with them! You'll end up paying a bit more, as they don't do negotiations. But honestly it does take a bit of the headache out of buying. Also, they don't sell every car they buy, all the cars on their lot actually have to pass a bunch of tests for them to sell it. Many cars they buy go straight to auction because they don't pass. I feel like the cars there are definitely more reputable than from private parties or mom and pop used car dealers. They may be better than some full dealerships as well. They also offer extended warranties that actually work when you need it down the road if that's something you like to have.
Also, a little known Carmax feature is that you get 7 days to return your car for a full refund no questions asked. I think this is great, as you'll really get to figure out if your car has some quirk or noise you didn't notice at first once you start daily driving it.
Bought from a Chevy dealer in Northern KY Tom Gill Chevrolet. I was replacing my totaled 2011 and looking for another stick shift and they happened to have a left over 2012 at the beginning of 2013 besides I was using my GM Card rewards so I needed/preferred to buy new. But I have good things about Carmax just never shopped there. Buying a car is always a difficult task but if they have what you like and as per what au201 says. Sounds like a good choice to shop there. Just get whatever you decide checked by your own mechanic not theirs!!!
Good Luck! BTW what kind of car you looking to buy?
We looked at CarMax almost 10 years ago. If anything, it let us compare a couple different vehicles we were researching as a family (Highlander, Pilot, MDX). Ended up liking the MDX, but not the one they had on their lot. They do tend to be marked up a bit, but what they're selling is usually a pretty decent all around vehicle.
My own used cars have always been bought from private parties, but sometimes you don't know quite what you're getting (bought a Honda when I was 16, turned out to need a LOT of work).
The black Cruze was bought new from Hendrick Chevrolet. Didn't much care for the dealer nor the salesman. The blue one was from Koons. Great salesman and great incentives on the car.