D.C. Mun. Regs. tit. 9, r. 9-451
If a motor vehicle or trailer has extra equipment or accessories attached to it at the time of sale, that extra equipment or those accessories shall be considered a part of the motor vehicle or trailer (the sale of which is exempt from the sales tax), and the sales price of the extra equipment and accessories shall be included in the fair market value of the motor vehicle or trailer for purposes of determining the D.C. Motor Vehicle Excise Tax.
If a motor vehicle or trailer is sold to a non-resident and will not be titled in the District, the receipts from the sale of the extra equipment and accessories attached to the motor vehicle or trailer at the time of sale shall not be subject to the tax.
If a chassis is purchased from a District dealer or other person by a non-resident of the District and will not be titled in the District, and subsequent to the purchase of the chassis, a body or any other equipment or accessories are attached to that chassis, reimbursement for the sales tax shall be added to the sales price of that body, other equipment, or accessories if the purchaser takes delivery within the District. The receipts from the sale shall be taxable to the vendor.
D.C. Mun. Regs. tit. 9, r. 9-451