* China’s securities regulator, in its most drastic step yet to arrest a selloff on Chinese stock markets, banned shareholders with large stakes in listed firms from selling for the next six months. * About 30 percent has been knocked off the value of Chinese shares since mid-June, and for some global investors the fear that China’s market turmoil will destabilize the global financial system is now a bigger risk than the crisis in Greece. * Fed officials need to see more signs of a strengthening U.S.