A suburban Philadelphia real estate firm has paid $509 million for a large swath of Mark Center in Alexandria, one of several remaining assets The JBG Cos. sought to sell prior to its merger with Vornado Realty Trust's D.C. business to form JBG Smith Properties. The sale to Morgan Properties, based in King of Prussia, Pennsylvania, includes more than 2,600 multifamily units and the 63,320-square-foot Shops at Mark Center.