Oh. Try to list an auction. You will get 50 bonus credits instantly for listing an auction(as long as its allowed by the rules(http://www.listia.com/rules)). You also after the auction ends will get the credits from the highest bid.
If you don't want to do that, you can buy credits here --> http://www.listia.com/account/buy_credits
Agreed... Been saving my credits for this though so watch out guys... ;) Also do you have any 48 hour codes? Cos I'm willing to take this auction over 500 creds if you would throw like 2 of those in :D