(a) Principles applied. In determining the dutiable value of articles examined under § 148.23, the Customs inspector shall apply the principles of section 402, Tariff Act of 1930, as amended (19 U.S.C. 1401a), and shall not regard the declared value or price as conclusive.

(b) Adjustment of value declared. An adjustment shall be made by the Customs inspector whenever the purchase price or value declared differs from the fair retail value, whether by reason of depreciation due to wear or use, circumstances of purchase, or acquisition, or for any other reason. He shall give due consideration to the condition of the articles at the time of importation, but he shall not make any allowance for wear and use in excess of 25 per centum of the declared price or value of a worn or used article. A passenger who desires to claim a larger allowance may arrange for formal entry and appraisement of his goods.