Rather than putting someone's email address in the clear on a webpage, you might take another approach:

• Place a "Send email to Realtor John Doe" link that invokes a properly written mail form and CGI script, rather than actually giving out the email address. User could compose an email to the realtor requesting his email address personally, if needed..
• Place a "Send me this realtor's contact info." link. The user would provide an email address to which the realtor's contact info (including email address) could be emailed.
• Ask the end user establish an account and to log in, providing a verifiable email address before gaining access to the database.

As long as you put an email address in the clear (or even as a hidden field in a form), it can be harvested.

Update: Woops, not discussing email address harvesting are we? Well, the principles I mentioned will also work for real-estate listings. Just keep the address confidential; require that the user send a message to get the address, etc. Require user log-in before addresses can be obtained. Authenticate new users by requiring valid email addresses, and so on.