This is what you shouuld know
1. The cron task is what expires ads and ends auctions.
2. The users get the appropriate emails of who won the auction or if an ad is expired to renew.
3. The user can renew his ad from the link sent in the email or if he views his ad under the ad options.
4. If expired ads are not renewed and over 2 weeks old the system will automatically delete them when you go to admin panel