First off, I think just putting together a marketplace for experts isn't enough, I've seen one of such marketplaces and it didn't have throngs of students as you would expect. Really going all out with the marketing and not expecting that because these experts have some form of fame and influence that would impact your marketplace.
The second thing I think could be missing is more like a mindset thing, where people believe, if you're already an expert that means you may not totally get me, understand where am coming from or just simply you're already rich, why would you take my little $$$.
This I've describe may not affect all cultures but it does affect some. Hope others weigh in. It would be interesting to see people's thoughts on this one.