The Bangor Mall may be up for sale at auction, but other retail real estate in the Bangor area is flourishing, with vacancies at 10 percent and below in downtown and around the mall. “The downtown market has a small vacancy rate,” Bev Uhlenhake, an agent at Epstein Commercial Real Estate in Bangor, told about 1,000 attendees at the Maine Real Estate and Development Association’s annual forecasting conference Thursday in Portland.