The big question is can you offer financing? If the answer is yes, you should sell it on your own as you will have a greater profit. If you can’t offer financing, than sell it to CarMax. Trying to get more than that out of a private seller on marketplace will be tough because most people don’t have that kind of cash and will need to finance it anyways. People feel better about financing through a dealer than a private seller, so above a certain price point on the marketplace will just sit. So take the CarMax money and run.