I sold to CarMax as well. Neat job, only took me an hour or so for the full process. They handled everything from evaluation to document processing etc.
Quick note - Keep your sales docs handy with you, you'll need it later. In my case, the guy who bought my car went on to get tickets from toll roads, while I was in India, and when I called up toll service to claim dispute, all they wanted was proof of sales. I had to email it to them.
Let us know how vroom handles your situation.