If a user doesn't have an outstanding balance, the UI doesn't show this (to avoid unnecessary confusion).
Aside from the UI displaying this information, when an auction closes, the winner receives an email that notifies him to add credit to his account.
Additionally, the system emails every user with an outstanding balance multiple times before the 2 week payment period expires. After this payment period expires, the auctioned domains are released to be re-auctioned, and the user is charged $100 or 2% of the winning bid for each domain that wasn't paid for (whichever is larger).