We have done it a number of times---both the way SnowCub described, and with the aid of a car trailer.
Very simple:
1. Go see the car with the car trailer in tow (the kind where the new car's wheels are ALL off the road so you can run it without plates)--drive the car and decide if you really want it.
2. If so, have the owner sign the title over to you--filled out completely and accurately.
2. Write up a complete bill of sale--at least 2 copies--and all people* sign it.
*All names from the title must be on the BoS with signatures and dates. Real legal like.
3. Call your insurance co and tell them that you are buying this car. Have the VIN, and pertinent details. IIRC, my insurance co covered the car from the end of the call.** They'll fill out an RMV-1 form for you.
** not necessarily of you
4. Take the bill of sale (make copies, they take all the papework), a checkbook or cash, RMV-1, and completed title to the Mass registry, sit for a couple hours. You pay tax based on the RMV's assessed value and get your plates, registration, etc.
5. You have up to 7 days to get it inspected--including your registry day.
6. Many happy miles and GTGs follow!
* Not the case if you use the SnowCub method of transport. *