I think it really boils down to if it's a generic car or an enthusiast car/performance car. I have used carmax both for enthusiast cars and generic cars, and it's been pretty fair for appraisals on generic cars yet hideous on enthusiast cars.
for example, my family has sold 2 generic cars to them, a 2006 chevy malibu with highest trim and only 40K miles, and a 2012 Honda element. Both automatic. Each time they got the KBB retail price or even slightly over with the malibu.
Enthusiast wise, I took a highly modified Mazda speed3 to them just for fun and it was hilarious. It was a 2008 I believe, 66K miles, had just about every mod you could think of including an upgraded turbo, megan racing coilovers, turbo timer, enkei rims, and was pushing nearly 350 WHP ... to the front wheels
and they ended up offering me $4700 or something like that. this was 2 years ago so Im not sure how the car has depreciated now but at the time i was looking for about 10K. I sold it privately about 2 weeks later for 11.5K