The tax imposed by this chapter shall not apply to the following:
(1) Sales upon which taxes are properly collected under Chapter 20 of this title;
(2) Sales exempt from the taxes imposed under Chapter 20 of this title;
(3) Sales upon which the purchaser has paid a retail sales tax or made reimbursement therefor to a vendor or retailer under the laws of any state or territory of the United States; and
(4) Sales of material or equipment used in the construction, and materials used in the repair or alteration, of real property; provided, that the materials are temporarily stored, for no longer than 90 days, in the District for the purpose of subsequently transporting the property outside the District for use solely outside the District.