Considered a reproduction and purchased for just $30, a drawing by German master Albrecht Dürer is now estimated at $50 million. It is currently being exhibited in London. In 2016, during a garage sale in Concord, near Boston, a bargain hunter stumbled upon a design bearing the famous monogram of Renaissance artist Albrecht Dürer. The sellers, who had inherited the work, assumed it was a modern reproduction and sold it to him for $30. Three years later, Clifford Schorer, an associate at Agnews Gallery in London, heard of the story during a stay in Boston. He immediately knew he had...