The best way to get the most credits is to give away the stuff you no longer want/need. Find something around the house or office that you don't need anymore and 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 bonus credits every time, and there is a limit on how many you can get in one day. The more active you are, the more chances you have of getting bonus credits.
Find more ways to earn credits here.
You can also view all your credit activity here.