With the development of a new mobile payment system, a group of retailers led by Wal-Mart Stores (WMT.N) is aiming to upend the $4.5 trillion credit card market and control the precious transaction data generated at the checkout line. The difficulty of the task became clear this week when drugstore chains CVS Health Corp (CVS.N) and Rite Aid (RAD.N), in a move apparently aimed at shoring up the retailers’ pay system, stopped accepting payments on Apple Inc’s (AAPL.O) iPhones.