Buffett has sold $7 billion of BofA. Is it a warning sign?Berkshire Hathaway CEO Warren Buffett has long been a fan of bank stocks. In fact, the banking/insurance sector has likely been the sector he's invested the most in over his career. In recent years, Bank of America (BAC -1.47%) has been Berkshire's No. 1 holding among bank stocks, but that seems to be changing.Buffett's conglomerate has been steadily selling off Bank of America, and it's not fully clear why. So what should investors do? Buffett and B ...