After twenty years of being a State Trooper I know some of my fellow Troops aren't gun nuts but they still take care of their equipment. They have to as it gets inspected regularly. With that I wouldn't buy anything used from a police agency. The stuff has a life cycle for a reason. To me buying a used pistol from a police department is like buying an old police car. What do you think you're going to get. Cops are hard on equipment because they use the hell out of it.
