Currently email alerts are only sent when a new listing is added. It would be great if email alerts would be triggered when Price changes and when a listings status changes from Sold or Rented to For Sale or For Rent.
If a listing is sold, or rented, and then later comes available again and its status changes to for rent or for sale, no email alert is sent, instead you would need to delete the listing and add it back into the system in order for it to trigger an email alert.
Price changes are quite common and should trigger an email alert as well.