Apart from CarMax, there are essentially two routes for selling a vehicle. The first is a private sale, which basically involves taking on a temporary, part-time job to have people meet you, test drive your car and then negotiate for the correct price. Craigslist makes this easier, but it's still intense. I'd love to see hard data on the amount of effort involved in private sales, but from anecdotes from friends who have done it, a person can easily invest tens of hours in attempting to get the vehicle sold on the market. That adds up to $1k fairly quickly. Private sale makes the most sense as a hobby. (I'll note that some startups like CarLotz exist that basically have a consignment model for private sales that may work slightly better in certain circumstances.)

The second route is the "trade-in." Most dealers almost never pay cash for the amount of money that they offer for trade-in. It's purely a negotiating tactic meant to make a buyer feel better about a (relatively arbitrary) price on a car ("$14k for a 2015 Ford Focus is kind of high, but we really like that we're getting $4k for our 2007 Pontiac Aztek!"). Whenever you work with a dealer, you're almost always getting fleeced. Again, some people enjoy the process as a sort of game or hobby, but you're likely not making a profit unless you're extremely good at it.

EDIT: I read this over and it sounded a little fanboyish. I want to make clear that I've been gone from CarMax for almost 5 years and I don't own stock or have any other financial stake in their success other than some friends who are still employed there. I just think they're a good company (although I didn't love working in IT).