18- 125 C.M.R. ch. 318, § 4
The sales and use tax exemption for instrumentalities of interstate or foreign commerce applies only to the purchase of vehicles, railroad rolling stock, aircraft, and watercraft. Repair parts, operating supplies, and accessories are not exempt, regardless of whether they are purchased at the same time as the vehicle, railroad rolling stock, aircraft, and watercraft or purchased separately. Accessories purchased as part of a vehicle are exempt from sales or use tax if the vehicle qualifies for exemption. A so-called "glider kit" is considered a repair part, and its purchase is subject to tax whether or not the vehicle on which it will be mounted is used by the purchaser as an instrumentality of interstate or foreign commerce.
18- 125 C.M.R. ch. 318, § 4