This is why I explicitly brought up answer sites. Your idea doesn't get the benefits of those kinds of sites, because publishing the expertise limits your ability to sell it (and makes it into another answers site). If you don't publish it the site does not become incrementally more valuable every time a question is answered, as Stack Overflow or Yahoo Answers does. It always remains a place where someone can buy expertise from someone they don't know, with qualifications that may or may not be accurate, for a price that cannot be compared.
My point is that your proposal seems like it would have all of the negatives of starting a new answers site, but none of the long term benefits. Answers sites start off with these problems, but may eventually overcome them as more and more questions get answered and the pool of regular readers and submitters grows, but yours would have these problems forever. Your site doesn't get bigger, stickier, and more powerful, when you sign on new users and new experts.
I'm not saying you can't do it, or that it won't be successful. I don't know. As Paul Buccheit says:
Limited Life Experience + Overgeneralization = Advice
I simply don't see how you (or even Google, who have retired their own Google Answers) could overcome those barriers and reach a self-sustaining business.
As someone that connects industry specific knowledge to people that need knowledge, and perhaps has tools that make that task easier, you could very well have a lucrative consulting gig. But, I don't know that you'd have a startup.
But, what do I know? I build systems management software.
Now as far as this particular idea on selling experience, I would imagine the traffic and activity would start from the ones interested in selling the experience. The interests need to start with people with valuable experiences who may show interest in trying out the service to see just how profitable they could be by sharing their unique experience. Then this can spark interest in the "buyers" who may find the right experience/knowledge they were looking for.
If you get the right distribution, I believe it can overcome the challenge you speak of. The question I would ask you is: "If Google launched this service tomorrow, would you think they can make it successful?"
This said, I would agree that it would be a challenging start-up if you do not have the right plans/capability to scale up.