An anonymous Londoner may have scored the deal of a century at a local flea market. In the 1980s, the unidentified woman bought what she thought was a gaudy costume ring at an outdoor second-hand bazaar for 10 pounds — the equivalent of $13. She wore the ring every day, and never thought much about it until a local jeweler told her it might be worth something. It turned out to be a vintage 19th-century ring set with a genuine 26-carat diamond, and the piece is now expected to fetch up to $450,000 at an upcoming Sotheby's auction. "This is a one-off windfall," says Jessica Wyndham of Sotheby's, "an amazing find."