Mass. Gen. Laws ch. 64H § 27A
Where a trade-in of a boat is received by a dealer in boats holding a valid vendor's registration, upon the sale of another boat to a consumer or user, the tax shall be imposed only on the difference between the sale price of the boat purchased and the amount allowed on the boat traded in on such purchase. When any such boat traded in is subsequently sold to a consumer or user, the tax provided for in this section shall apply.
Where a trade-in of an airplane is received by a dealer in airplanes, who has registered his federal dealer's aircraft registration, upon the sale of another airplane to a consumer or user, the tax shall be imposed only on the difference between the sales price of the airplane purchased and the amount allowed on the airplane traded in on such purchase. When any airplane traded in is subsequently sold to a consumer or user, the tax provided for in this section shall apply.
Mass. Gen. Laws ch. 64H, § 27A