Very tricky problem to solve and must say an innovative idea to solve it. Some points to be careful about:
1) Be sure that the buyers are happy to take their requirements public (or to another site). Sometimes even the site that currently hosts the requirements might have objections to being crawled
2) In case of matches (buyer and seller) woudl you be emailing them both - be careful about getting some optin from each of them prior to emailing them the opportunity to transact.
3) Would recommend emailing the people being listed on both side of the market in advance and as soon as you list their information and provide them an easy to execute opt out option
4) And then there is the regular challenge of doing too much for free at the start making it difficult to bill the same later...