AT&T retail stores will be offering refurbished iPhone 3GS models to its prepaid GoPhone customers — no-contract accounts that do not qualify for subsidized phones — according to a MacRumors tipster. The iPhone 3GS was originally released in 2009 and, notably, will not run the new iOS 7 when it’s released this fall. For comparison, AT&T sells the refurbished iPhone 4 for $325, so the 3GS would presumably be offered for well…