The pilot reported that he parked the airplane and shut down the requisite systems, then proceeded to the back of the airplane to allow the passengers to disembark and set the wheel chocks. He reported that as he began to open the cabin door, he realized that the airplane was rolling backwards. The airplane rolled into the wall of the airport terminal and the left and right elevators sustained substantial damage. The pilot reported that there were no mechanical failures or anomalies with the airplane or any of its systems that would have prevented normal operation. According to the manufacturer's pilot operating handbook and the operator, the accident airplane was equipped with a parking brake. The operator reported that their maintenance department inspected the parking brake system after the accident, and in accordance with the Beech B200 service manual and found no failures or anomalies.