A big (maybe biggest) reason why buyer initiated bounty markets don't work is that the buyer is shouldering the initial cost and risk. They have to take the effort to *maybe* get a result, when they're the one trying to spend money and want to be as lazy as possible.

In contrast, if they go to a seller-driven market ("I will sell this for $50") then they know those opportunities are ready for their taking.

The markets where this works are the ones where the demand-supply balance is so wonky and/or so liquid that a buyer is sure to get responses from sellers. Examples would be oDesk/elance/99 Designs, where people post "I want to pay $100 for xyz."

You could count the stock markets too if you include bid-ask systems.