This glitch utilizes the "get any pack for 8000 credits" glitch. If you don't know how to do that, click here or watch the video below.
The pack store always has one pack that gives a specific set of cards. Examples of these packs are the Femme Fatale Pack, The Starter Pack and the Bleed Out Pack. If you do the 8000 credits glitch and constantly get these packs, you can sell all of the duplicates. Usually the cards in these packs sell for 150k credits total. You spent 8k credits on the pack, so you can keep repeating this glitch for unlimited credits.
If you want more credits, you can also do this with the most wanted pack. Once you've unlocked all of the characters from the most wanted pack you can usually sell the duplicates for over 150k credits (Shazam and AOBatman sell for 100k + each!) You can also get an unlimited amount of alliance credits by constantly buying the most wanted pack and selling the joke joker.