By playing the story missions in Darksiders Genesis, you will unlock access to shops. There, you can buy potions, supplies and skills.
All unlocked stores are in the Void. You reach this place after completing a chapter.
However, if you are concerned that stores can only be found in the Void, we have good news for you. All unlocked stores can be accessed via Summoning Stones.
Summoning Stones also allow you to play in the split-screen co-op mode.
If you want to buy an item, go to any Summoning Stone.
The store will never close, so you can keep coming back to it and restock.