Each wholesale dealer or jobber of lumber and timber and each wholesale dealer in lumber and timber on commission whether maintaining an established place of business or not shall pay a license tax of $100. A sawmill operator, regularly licensed under Section 40-12-154, shall not become liable for the license tax imposed by this section by reason of his purchasing partially manufactured lumber from other sawmills, if the processing of said partially manufactured lumber is completed at the plant of the sawmill operator so purchasing the same and the lumber is thereafter shipped or sold in the same manner as lumber manufactured at the plant of such operator; provided, that such purchases do not exceed in volume the lumber manufactured by such operator at his own plant or plants.