Investor Warren Buffett's net worth has surpassed $81 billion and yet, for all his business success, the self-made billionaire still regrets some missed opportunities. One of them is not investing in Amazon when he had the chance.

At Berkshire Hathaway's annual meeting in May, Buffett said he wished he had chosen Amazon as his value tech stock instead of IBM. As he then told CNBC, he was "too dumb" to have seen the potential in the e-commerce giant.