Offices or office building for administrative, executive and professional activity, and similar activities involving the performance or rendering of professional service, such as a bank, financial institution, the sale of real estate and business office including the showing of samples, sale promotion and demonstration of equipment; provided, however, that no merchandise shall be warehoused on the premises for sale, exchange or delivery thereon, and further provided that nothing herein preclude sale or manufacturers' representatives from arranging for the sale of merchandise manufactured, fabricated or warehoused at, or delivered to, locations outside of the PI District.