i sold my car to them 3 weeks ago.
didnt want to deal with having to list my car on craigslist and have people "interested" , then do test drives, etc then try to haggle me down while having to own 2 cars. the carmax price honestly was fairly competitive with what i saw on craigslist minus the usual haggling. maybe i could have gotten another $1000 on craigslist maybe not. but i had sold a car on craigslist before (a much less expensive one , and it was already kind of shady the way it went)
That and my car (2010 Audi A4) was pretty expensive and had some minor reliability issues so well let carmax deal with it and not have some guy come after me when some minor issue comes up who has my phone number etc.
anyway, you go to carmax, they look at yoru car and give you a quote. you get to keep that for 7 days (and they dont seem to care if you put some miles on it in that 7 days, at least they didnt care that i drove from LA to vegas in my car and put an extra 750 miles or so on it, i even asked them this when they gave me the quote).
so 7 days later, i was able to get the car i was buying and just had the dealer of the new car pick me up after i dropped off my old car at carmax. you can drop off your car at any carmax once you have a quote.
it was actually really easy, they gave me a check after verifying i had the quote etc. the dealer i was buying a new car for , said on blackbook or whatever they use for auctions that they could only give me $1500 less than carmax (19.5 vs 21 in this car) and that even they sell their own personal cars to carmax.... so yeah i dont know seemed like an ok way to sell a car to me.
i ended up basically getting "good kbb trade in" vs "fair" which is what the dealer would have given me (i'd say realistically my car was "good" not "very good" but definitely better than "fair"). based on values in the area for my car, i really dont think the private party value is all that accurate, but who knows.