Old man Crazy Earl has set up a secret shop in Borderlands 2 somewhere in Sanctuary where he’ll reluctantly trade inventory-increasing Storage Deck Upgrade units for Eridium crystals. Just don’t tell anyone where you got them.
Where to find Crazy Earl's Black Market in Sanctuary[edit]
The location of Crazy Earl's Black Market is near Moxxxis. Look for a pink/purple vertical sign that says "HOSTEL". The door underneath that sign is the door to Earl's Black Market. See picture below: