I just had my first experience with this last week. I had a 20 year old vehicle that I did not feel comfortable selling to an individual.
My mechanic who knows the car well offered me $1500 because he felt he would have to do significant work to the suspension. The online valuation from Carvana came back $2700. The local giant preowned sales dealership offered me $3000. CarMax bought it for $5000.
I learned several things. With Carmax it doesn’t matter if you do the online evaluation or take the car in to the dealership. The number is exactly the same. Then, when you are given an online offer and take the car in for appraisal, they basically verify every feature that you claim is on the car and if it is there, then they pay you the online offer. The process was very slick and very organized, and very automated. About two hours after I did the online assessment, I had a check and the car was sold.