It shall be the duty of every operator required to make a return and pay any surcharge under this chapter to keep and preserve suitable records of the gross daily rentals together with other pertinent records and documents which may be necessary to determine the amount of surcharge due hereunder, and such other records as will substantiate and prove the accuracy of such returns. All records shall remain in the city and be preserved for a period of three (3) years, unless the treasurer, in writing, has authorized their destruction or disposal at an earlier date, and shall be open to examination at any time by the treasurer or by any of his duly authorized agents. The burden of proving that a sale was not a surchargeable sale shall be upon the operator who makes the sale. (2000 Code § 7-710)