if the item expires, it is gone. if the item is sold, it's still there. what we want from etsy is an automatic renewal system where we can have the option to have our items renew on their own if they haven't sold yet.
i wish sellers would contact me if they renew their item so i can add it again. however, are you sure it doesn't just pop back up after the renewal. i have never looked in to that before. it just happened to me the other day, i was reading comments on a treasury i made for a challenge and one of the items was gone. i had to find something else to put in its place.
good luck!