ยท How and when do I have to run the cron.php script?
cron.php is responsable for closing expired auctions, determine if there's a winner and send notification e-mails to seller and (eventually) to the winner.
For your site to reflect a "real" situation you should run cron frequently, let's say every 5 minutes (or less).
Under *nix systems this can be easily done setting a cron job which executes cron.php.
If you are not familiar with cronjob you may want to read the following:
What is a cronjob, and how do I use it?