Tradesmen such as plumbers, electricians, carpenters, heating and air-conditioning repairmen, who are resident occupants of the principal building may conduct their business from the premises; provided however, that no supplies or inventory shall be kept, maintained, stored or repaired on the premises and that no more than one truck (panel or pick-up) of not more than 8,000 pounds GVW may be parked, stored, or garaged at any one time upon the premises.