There are several companies that buy old barn wood, including Pioneer Millworks, Mountain Lumber Company and Longleaf lumber. These companies request that you send them an email with specific details and photos of the barn wood so that they can decide whether they want to buy it.
These companies do not dismantle the barn for you, but they do purchase the wood after it has been carefully dismantled and checked to ensure that it remains in good condition. The company's decision may also rest on the type of wood. Most companies that purchase antique barn wood want White Oak, Red Oak, Chestnut and Hickory.