Customers who were buying even as gold began slumping in 2013 are now so scarce at the Bullion Trading LLC shop in New York that owner Isaac Kahan says sales in May tumbled 35 percent. Purchases of American Eagle gold coins from the U.S. Mint, the world’s largest, were the weakest for the month in eight years. And global coin demand this year probably will slump to the lowest since 2008, TD Securities Inc. predicts.