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
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: