Hello @LeticiaG731 - Since Constant Contact is a permission based marketing company, we do have some general rules around the types of business relationships that count as permission. I'd encourage to read this article to learn more about who can and cannot be emailed to through Constant Contact.
If you do have a business relationship with realtors that have opted to receive your listings, you can upload them to a list of Contacts. This can be done by integration, or commonly from exporting a spreadsheet from where they are currently store and importing it into Constant Contact. Here is more information and instructions on importing contacts.