(5) Upon every person, firm, or corporation engaged or continuing within the county in the business of selling at retail any machine, machinery, or equipment which is used in planting, cultivating, and harvesting farm products, or used in connection with the production of agricultural produce or produce, livestock, or poultry on farms, and the parts of such machines, machinery, or equipment, attachments and replacements therefor which are made or manufactured for use on or in the operation of such machine, machinery, or equipment, and which are necessary to and customarily used in the operation of such machine, machinery, or equipment, an amount equal to one-fourth percent of the gross proceeds of the sale thereof. Provided, however, the one-fourth percent rate herein prescribed with respect to parts, attachments, and replacements shall not apply to any automotive vehicle or trailer designed primarily for public highway use, except farm trailers used primarily in the production and harvesting of agricultural commodities. Where any used machine, machinery, or equipment which is used in planting, cultivating, and harvesting farm products, or used in connection with the production of agricultural produce or products, livestock, and poultry on farms is taken in trade or in a series of trades as a credit or part payment on a sale of a new or used machine, machinery, or equipment, the tax levied herein shall be paid on the net difference, that is, the price of the new or used machine, machinery, or equipment sold, less the credit for the used machine, machinery, or equipment taken in trade; and