Purchased 5 items from the black market

The black market can be found in the Sanctuary, all the way on the east side of the map. Youll have to visit this shop when doing a main story mission. The shop is also marked on the map, like any other. The salesman is behind a door. Youll need to purchase 5 items from him. These items increase your ammo limit, inventory size etc

Instead of paying with money, youll have to pay with Eridium. The total amount of Eridium you have can be found by opening your menu, go to weapons and look at the bottom of the tab. Its the big purple E. Youll need at least 20 Eridium to buy 5 items from the market

Eridium can be found by opening chests, loot from enemies, mission rewards etc. Theyre purple when dropped and youll have to pick them up yourself, unlike cash