I have sold 3-4 vehicles through Craigslist. I worked up a bill of sale that lists all the pertinent info (car details, buyer, seller, sale price, etc.) and bring two copies to the deal so we can both sign and each walk away with a copy. Most of the deals are cash and I have no problem counting it in front of the buyer. We usually meet up, do a test drive and then either they buy it right then or we meet again after they thought it over and withdrew the cash. I usually do the exchange at a grocery store parking lot. Make sure to remove your plates and take them with you when you hand over the keys. One vehicle the buyer wanted to finance and that was more of a pain. I had to give him a bunch of info so he could get the loan through his bank and then we met and exchange the bank check and vehicle. I held the title until the check cleared and then mailed it to him. He was ok with that arrangement.
In general you will do MUCH better doing it yourself rather than trading it in or doing something like Carmax. I have taken vehicles to Carmax and then turned around and made 3-4k more selling them myself on Craigslist. You pay a lot for the convenience that Carmax provides. In my case it would have been thousands. Horrible value prospect given how easy it is to take a couple of pictures and meet a guy in a parking lot.