There are plenty of items to load up in your shopping cart at Costco -- but gold bars shouldn't be one of them.From 50-pound bags of rice to pet insurance to solar panel installation, the range of items and services that Costco (NASDAQ: COST) provides its customers is remarkable. One of the most popular items recently, though, is gold bullion. Of the 101 stores that Bloomberg surveyed earlier this month, 77% reported that they had sold out of bars of the yellow metal. But buying bullion only will translate ...