Ala. Code § 40-23-37
There is hereby levied, in lieu of the state sales tax levied by Section 40-23-2, a privilege or license tax against the person on account of the business activities engaged in and in the amount to be determined by the application of rates against gross sales, or gross receipts, as the case may be as follows:
Upon every person, firm, or corporation engaged or continuing within this state in the business of selling at retail any machine, machinery, vessel, or equipment which is used in planting, cultivating and harvesting farm products, the capture, attempted capture, or processing of fish or other seafood by means of commercial fishing by a holder of a commercial license issued pursuant to Chapter 12 of Title 9, or used in connection with the production of agricultural produce or products, livestock or poultry on farms, and the parts of such machines, machinery, vessel, or equipment, attachments and replacements therefor which are made or manufactured for use on or in the operation of such machine, machinery, vessel, or equipment, and which are necessary to and customarily used in the operation of such machine, machinery, vessel, or equipment, an amount equal to one and one-half percent of the gross proceeds of the sale thereof; provided, that the one and one-half 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, vessel, 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, vessel, or equipment, the tax levied herein shall be paid on the net difference, that is, the price of the new or used machine, machinery, vessel, or equipment sold, less the credit for the used machine, machinery, vessel, or equipment taken in trade.
Ala. Code § 40-23-37 (1975)