Costco built its brand on supermarket staples. If you wanted a truckload of toilet paper or enough ham to pack 50 sandwiches, it was the place to go. But Costco has increasingly moved beyond mustard and milk.The most well-known "surprise" in Costco's sales success strategy may be the boom in gold bars bought by its customers in recent years. But the big-box retailer is also seeing a boom in sales of saunas and sofas, many of them purchased online. In industry parlance, these purchases are termed "big and bu ...