Office or office building for administrative, executive and professional activity and similar activities involving the performance or rendering of professional services, such as financial institution, mortician and 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 except for sale, exchange or delivery thereon, and further provided that nothing herein shall preclude the sale of merchandise manufactured, fabricated or warehoused at or delivered to locations outside of the Central Business District.