For purposes of the tax imposed by section 4061(a)(1) (relating to trucks, buses, etc.), in determining the price for which an article is sold, the value of any previously used component of such article shall be excluded from the price if the person furnishing the component is the first user of the finished article. For example, where a manufacturer builds a truck for a customer who intends to use, rather than resell the truck, incorporating used parts furnished by the customer, the value of the previously used parts shall not be included in the price for which the truck is considered sold by the manufacturer.