When buying a used documented vessel
 

Do you even have to register it with a state or for that matter pay the sales tax?

u buying a lightship?

from --> http://www.mass.gov/dfwele/dle/boatregfaq.htm

Am I required to register my boat?

State law requires the registration of any boat that is powered by a motor and operated on public waterways in Massachusetts. Registration is required even if the motor is not the primary means of propulsion for that boat. Some examples of boats that require registration include fishing boats with motors, recreational motorboats, canoes or sailboats that use motors (includes electric motors), and personal watercraft such as Jet Skis or wet bikes. Boats exempt from registration requirements include those that do not use motors, and documented vessels (large boats that are issued a marine document and registration through the U.S. Coast Guard). Vessels used solely by a city, county, state, or federal agency will be issued a certificate of registration and number at no charge.

Sweet!

No lightship. (But my wife would like that kind of craft)

I am going to look at a boat this week and it is documented. I was just crunching some numbers and was wondering about the MA tax issue.