@r.mutt-1917
Unfortunately I do not think there is a way to undo the deletion of items from the Watch List.
The only way to rebuild the list is to recreate the searches and add individual items back to the list one at a time.
If the items were recently added you may be able to find some items in your Recently Viewed List, or among your "Saved For Later" items on your cart page, or on your Saved Searches or Saved Seller Lists.
You may be able to recreate some searches using your browser's search history, or the saved form data from the Advanced Search page you can recover by clicking on the search field there.
