This is actually an old offline business model, you might get some ideas from "exclusive business networking." It was and may still be particularly big in real estate sectors, where an agent can "buy" referral rights for a given community. Participating realtors refer clients leaving *their* community to the destination community realtor. Assembled with it, realtors can have their own groups of business categories: homeowners need movers, plumbers, roofers, etc, all exclusive to that realtor in terms of referrals. So the referrals expand to other business groups as well.
The lead or referral is the "gold" or currency, if you will. I'm no expert on this but privy to a client who has this in place, cannot comment on it's success or failure.
a complete listing of all the professional service providers in their area
First you have to understand, it wouldn't be the "clients" using these sites. It would be the businesses.
This is where they collaborate, build their network, exchange client leads. True, SOME clients would go there, but they'd likely go there only because their realtor or other rep told them to: "check out my list of contacts on this site. These are all people I would trust to give you the service you need."
In the context of the above scenario, I'm moving from L.A. to Washington D.C. I have no idea who to trust, I can just as easily pick up a phone book and play roulette with realtors as dig through an Internet listing. My current realtor is part of this network, and hooks me up with the D.C. realtor in the network. When I get there, he has all the contacts I need to find a home, get moved, where to go for this or that.
Why should we trust those within the network? Because for it all to work, each participating member is motivated to select associates they trust to fortify their reputation.
It's a pretty good concept, whether it works or not . . . can't say.