National Motorists Association
Massachusetts Page

When you are registering a used car you bought from a private person, the state sales tax will be based on the value of the car according to the NADA Official Used Car Guide - not on the negotiated price.

Under state law, officials can accept affidavits from private individuals who swear they gave a car away as a gift. Then, the lucky recipient doesn't have to pay a dime in state taxes.

Actually, there are two different laws:

• You can transfer a car to a close relative (which means give or 'sell'), and they don't have to pay sales tax:
  http://www.state.ma.us/rmv/regs/reg3.htm

• You can give anyone a car as a gift, and they don't have to pay sales tax. You have to sign an affidavit (an MVU-24) where you swear (under penalty of perjury) that no money changed hands. I would expect some number of these get investigated.
  http://www.dor.state.ma.us/forms/mvu/pdfs/mvu_24.pdf