Michael Sassano turned his knack for mutual fund market-timing into a $10 million-a-year business for himself, people who know him say, and now federal regulators are broadening their scrutiny of the Oppenheimer & Co. ( OPY) stockbroker. In his best years, Sassano's annual trading commissions routinely exceeded $10 million and sometimes approached $15 million, according to people familiar with the 33-year-old broker. The money helped pay for a lavish lifestyle that included a villa in St. Tropez, where Sassano frequently vacations. To put Sassano's riches in perspective, the notorious group of former Prudential Securities brokers facing civil fraud charges raked in about $3 million in commissions from all of their market-timing activities put together, according to regulators. "A broker doing $10 million in commissions is phenomenal," said Jonathan Kord Lagemann, a securities attorney and a former general counsel for a brokerage firm. While it's not known what percentage of Sassano's commissions came from market-timing, at least one person estimated that just over half came from placing such trades for his long list of hedge fund customers. Market-timing is the term for a legal but strongly discouraged trading strategy in which mutual fund shares are bought and sold frequently in order to capitalize on price discrepancies in different markets. The rapid-fire trading is harmful for the vast majority of mutual fund investors because it can dilute the value of a fund by driving up trading and administrative costs. Most fund companies disclose in their prospectuses that they try to ferret out and stop market-timers. But investigators have found that far too many fund companies were willing to bend or ignore those rules when it came to a privileged group of hedge funds and their brokers. In a few instances, such as the former Prudential group in Boston, brokers have been charged with fraud in the burgeoning investigation into the $7 trillion mutual fund industry. (Prudential Securities is jointly owned by Wachovia ( WB) and Prudential Financial ( PRU)).