Kaja Veilleux has hunted New England attic treasures for more than 50 years. He once found a copy of the Declaration of Independence sitting on a pile of trash, and he made headlines this year when he stumbled upon a million-dollar portrait gathering dust in an old farmhouse in Maine that may have been painted by the Dutch master Rembrandt. Then there was the time, Veilleux said, he was shown a $50,000 gold coin kicking around in a tool drawer, only to have the well-meaning owner destroy much of its value before he could auction it by using a scouring...