Well, I have sold two cars to CarMax and their offers were at least reasonable. One was a 2010 Accord sold in 2013 and their offer was much higher (by thousands) than the offers from a couple of Honda dealers and a smattering of other new car dealers. I probably could have done better selling it privately but didn't want to deal with the hassle at the time. The other car was a POS 2003 Civic Hybrid with a failing IMA pack and an iffy CVT which was sold in 2019. Amazingly they gave me $1K for it. Amazing because as far as I could tell it had net negative worth.
Highest prices I see are used car dealers, so if you had a friend with a business like that who owes you... The other method I saw once is a guy made sure every tiny detail of his Gen2 was absolutely perfect and put it on Craigslist with high price saying on a specific day at a specific time they would accept the highest offer above that amount and show the vehicle to that person and if they saw something they didn't like, they'd go to the next highest bidder. That person got a super high price because they didn't forget a single detail, which assures the buyer the Prius was well cared for. I'll always remember that post!