I found a few helpful web sites with info on selling a used car. The Car Guys have a few pages that break it down, step by step, that was particularly helpful. Also the Rocket Lawyer has a place where you can get a free Bill of Sale document.
The Car Guys reminded me that I should call the Massachusetts Registry of Motor Vehicles, just to make sure I'm clear on what needs to happen. They have a ton of info on the web site, but I just couldn't find the answers I needed, so I tried by phone. I had to wait on hold for about 20 minutes, after twice getting lost in the phone maze (calling it a tree seems too kind), but I finally did get to talk to a human. She was both friendly and helpful and confirmed what the Car Guys had written, that once we make the sale happen, getting signatures and copies of the Bill of Sale and signed Title, we need to keep the license plates (I'd forgotten about that part) and then turn them in at an RMV office. They'll give us a Plate Return Receipt, which we will then give to our insurance company, and we can also use it to get back a refund of the excise tax we paid this year on the car.
Now we just need to get it to happen.