The city of Durham has closed on its purchase of a 4.5-acre block on East Main Street that will become the site of the new Durham police headquarters. The city paid $5.49 million for the property after additional environmental testing results came up sufficient, confirms Marcus Jackson, a Raleigh real estate broker with Trademark Properties who represented the seller in the deal. The property had been owned by an entity managed by heirs of the Carpenter family, who have owned the property since…