There are a few dynamics working in favor of us normal iOS users. While those that purchase the bug will have incentives to use it before Apple patches it, the odds are they will still restrict themselves to higher-value targets. The more something like this is used, the greater the chance of discovery. That also means there are reasonable odds that Apple can get their hands on the exploit, possibly through a partner company, or even by focusing their own internal security research efforts. And the same warped dynamics that allow a company like Zerodium to exist also pressure it to exercise a little caution. Selling to a criminal organization that profits via widespread crime is far noisier than selling quietly to government agencies out to use it for spying.
In large part this is merely a big publicity stunt. Zerodium is a new company and this is one way to recruit both clients and researchers. There is no bigger target than iOS, and even if they lose money on this particular deal they certainly placed themselves on the map.
To be honest, part of me wonders whether they really found one in the first place. In their favor is the fact that if they claim the exploit, and don’t have it, odds are they will lose all credibility with their target market. On the other hand, they announced the winner right at the expiration of the contest. Or maybe no one sold them the bug, they found it themselves in the first place (this is former Vupen people we are talking about), so they don’t have to pay a winner but can still sell the bug, and attract future exploit developers with the promise of massive payouts. But really, I know nothing and am just having fun speculating.
Oh what a tangled web we weave.