only legal way to do it is either FFL to FFL, or you bring it to a FFL (in the home state of the buyer) in ME or NH who is willing to do the transfer.
If you were in another state, the buyer could come to MA and do the transfer at a MA FFL, but the in aftermath of the 1992 Simon's Rock shooting, non residents can't buy a long arm in MA and take it home.... something you are allowed to do under Federal Law
Many states allow the sale/transfer of a long arm by a FFL to a non resident, providing the buyer is able to possess the gun in their home state
MA is not one of those states ( again Simon's Rock)