With WP Estate you can alert your users if a property that match certain parameters appears on your website. If this feature is turned on a “save search” form will appear on the page with advanced search results. A users can save it and create an alert that will send an email every time a similar listing appears.
The script that check for new listings and send the alerts can run every day or once a week. Also the users can see their alerts in their dashboard on the “Saved Searches” menu.