Through the first six months of the year, gas stations have made an 11-cent-a-gallon profit, but half of that has been eaten up by credit-card processing fees, says Tom Kloza, chief oil analyst at Oil Price Information Service in Wall, N.J. Meanwhile, retailers such as Costco Wholesale Corp., Wal-Mart Stores Inc. and even Home Depot Inc. have been building gas stations in their expansive parking l