Complex sentence: Convinced that he should change from brokering to investing, Cummings, starting in 1953, devoted his financial expertise to managing personal investments in the stock ticker and constant attention to the market, he exhibited a genius for buying and selling at just the right time.
Split into simpler sentences: Convinced that he should change from brokering to investing, Cummings, starting in 1953, devoted his financial expertise to managing personal investments in the stock market. For the next several years, with his own stock ticker and constant attention to the market, he exhibited a genius for buying and selling at just the right time.