Taskrabbit and oDesk.

Though over time the most common requests are usually standardized into products to reduce complexity for the buyer. For most buyers the open ended nature of a bounty system is intimidating.

Buyer Centric on it's own may work if the value proposition is strong e.g. Odesk, elance , every Job board under the sun (in some ways), etc.

Bounty based though is intimidating because of the process of defining rates is arbitrary, in a non f2f interaction it's much easier to have a pre-defined set of rules to engage in.

But there is a classic problem of chicken and the egg problem in a buyer-centric marketplace and if one does not think it through, you may be in for a very long haul.

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.

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