The best way to get the most credits is to give away your stuff. Find something around the house or office that you don't need anymore create a listing. You might also receive bonus credits for doing activities such as listing, bidding, commenting, leaving feedback and checking in. However, you don't receive credits every time, and there is a limit on how many you can get in one day. The active you are, the more chances you have of getting bonus credits.
Find more ways to earn credits here.
You can view the history of all the credits in your activity log by viewing the following link: www.listia.com/account/credit_activity