Billionaire Sam Zell is known as the "grave dancer" for buying distressed properties on the cheap.

That's how he made his billions.

So when Zell is raising alarm bells about the stock market, it's hard to ignore him.

"There's a significant and growing disparity between the stock market and the economy," he stated on CNBC.

"The markets do not reflect the risks we see out there."

Zell is only the latest expert to point out risks in the current market.

Noble prize-winning economist Robert Schiller has been shouting for months that stocks look overvalued.

No, they're not at dot-com bubble levels, but they are above average.