Through the blog for the bestselling book Freakonomics, by Steven Levitt and Stephen Dubner, we find Moneyball author Michael Lewis writing on Yahoo! Finance about stock markets in professional athletes.
Lewis cites the existence of Protrade, which is a faux market for athletes. You sign up for free and get a portfolio of fake money. You can then buy and sell (and short) the "securities" of the athlete in real time. Your "profits" can be turned into real prizes. As Lewis writes, there's a very short step from Protrade to an actual market.
Don't say this can't possibly happen: There's already a real-life hedge fund that invests in the transfer rights to soccer players overseas. David Bowie has sold bonds guaranteed by future royalty payments from his albums. Those bonds are asset-backed, which means they're not as risky as unsecured notes. But for someone looking for more risk, why not buy stock in the future earnings of Kevin Durant, provided he's willing to share some of the risk with you?