The concept of having a directory to get advice has been around for years .. earliest occurance I can find is 1999 with Ingenio.com (acquired by AT&T).
In todays world, there's www.healthtap.com | http://helpouts.google.com + many many others.
The key is community & liquidity. Marketplaces are all about building enough demand + business model to support growth.
It's not the technology, it's the implementation of that technology to ensure a great experience for both parties.
A lot of industry and professional associations I think would argue that they offer this through their membership and online communities and/or email listservers. It's not the same thing as Clarity obviously (I like Clarity better, frankly), but they certainly position it to be a place where you can get trusted advice from colleagues who have deep expertise in a particular industry or profession.