Unless you’re the world’s richest person, you shouldn’t be buying bitcoin. That’s the message from Bill Gates — the third richest.   With a rally of more than 400 per cent over the past year, bitcoin has become increasingly mainstream, and everybody, including prominent investors and policy makers, has been talking about it. The world’s on-again, off-again richest person, Elon Musk, recently invested $1.5 billion in the cryptocurrency through his company, Tesla, and said bitcoin would soon be accepted for payments.