Why don't you just sell it outright to a private party. Carmax is going to offer you nothing close to whats its worth, detail it, look it over and sell it for 12-25% more than they gave you.
When I was there, they had more sale men than customers! Just go in. Make sure to bring your documents and keys. You can get their price then post your car on the fb. Could get more money. Based on my experience, they were good and fast for buying my dad's car
You can just go in but an appointment will make things go smoother.
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