The S&P 500 currently trades at an expensive valuation that has historically signaled trouble for investors. Warren Buffett's Berkshire Hathaway (BRK.A -1.27%) (BRK.B 0.18%) held $344 billion in cash and equivalents in the second quarter, just shy of the$348 billion it reported at the end of the previous period. The company was also a net seller of stocks for the 11th straight quarter. History says the stock market is headed lower in the next one, two, and three years The cyclically adjusted price-to-earni ...