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).
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.
Absolutely. My expertise is in Analytics and the development of Business Intelligence Reporting Solutions for small-medium business. The tools themselves (and the selection of them) lead to lots of questions that work on this forum. The same is true for s number of other technical issues.
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.