Should another party be interested in the domain though, they can use the likes of Expirefish, SnapNames or one of the many similar companies who offer a domain 'backordering' service. If you're not familiar with this, these companies continually check to see if the domain has been made available for sale following expiry and try to purchase it if it's available.
If you were familiar with that bit then I apologise!
To go back to the original question though, your domain can only be purchased after it has expired and has been made available for sale again.
R.