This represents the "typical" relationship between a directory (hub/authority) and real estate agent sites. Some link back and some don't.

What we're gradually working toward is a closed network, where everyone that links to us receives a link back, and vice versa - concentrating almost entirely on real estate agent sites.

This helps improve not only our link popularity, but our status as an authority and the "relevancy" of both our incoming and outgoing links.  The more we are built up as an authority, the better for those sites we link to.