Hey Rich,
The most common way we used to do this was by forcing the customer to fill out a lead form in order to look at a product listing. Let's say that your realtor site has 50 listings on it and it shows a main picture with home location, etc....When the customer clicks the picture to view further, he will then be prompted to fill out a lead in order to proceed to the full product listing.
This will get you a good amount of leads and you can ensure they are good leads by promoting an authentication on their email, etc.