In my experience, buying a car out of state as a Mass resident is a huge pain. After you pay and get paperwork (bill of sale and title/certificate of origin), you can go to the DMV and register/get plates. After that you pretty much have two choices, either ship the car to you or make another trip to the seller with plates in hand to pick up.
I bought a 911 from a dealer in TX once when I was living in MA. After paying, they fedexed the paperwork and arranged the shipping from TX to MA. I registered the car while it was in transit. I paid ~$1300 shipping (in 2007), so $2k isn't unreasonable, depending on how far the car is being sent, especially if you don't yet own the car (if something happens during shipping, it's not your problem).
I've also traveled out of state with a car trailer in tow, and brought the car back (unregistered) in a single trip. But you need a trailer and something to tow it with (both/either can be rented).
I bought a car from a private party in NH once and made 2 trips (inspect, pay, leave with paperwork, then go back with plates and drive home). NH dealers near the boarder will often register for you. They won't issue a NH temp, as MA doesn't recognize any state's temp plates when driven by a MA driver.