With WP Residence 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.
screen capture with save search form
screen capture with search alerts in user dashboardÂ
screen capture with saved search theme options