Investors don't have to make guesses about where to put their money in the stock market. Billionaire investors conduct careful research into every stock they buy, which is why it's worth checking out what they are buying (or selling). Fortunately, investment firms that manage more than $100 million are required to disclose their holdings with the Securities and Exchange Commission (SEC). Three Motley Fool contributors combed through the holdings of a select group of billionaire managers. Here's why the star ...