Buffett might be worried about the valuations of stocks today.Warren Buffett has been sounding the alarm bell for quite some time now. Twelve quarters to be exact. That's the number of consecutive quarters that the billionaire has been a net seller of stocks, meaning his selling has outweighed his buying. On top of this, Buffett, as chairman and chief executive of Berkshire Hathaway, has been building cash to reach record levels -- in the third quarter, cash topped $381 billion.The famous investor hasn't ex ...