Whenever you visit any of the stores in Dragon Ball Z: Kakarot, they require you to pay for the items using Zeni, the main currency of the game. Unfortunately, a majority of the missions in the game do not reward you Zeni. They usually give you Z orbs, D medals, or ingredients to use to make high-quality meals with a cook.
The best way to make Zeni in Kakarot is to sell items you find in the world. These can be rewards you receive from side missions, hidden items you find while traveling the world, animal parts you receive after catching them, and many more. You want to make sure you’re selling valuable items, though, and not crafting materials you need to make recipes or create machines.
In the description, you can tell the above item; a Beast Horn can be sold to a store for a low cost. However, it doesn’t have any details that label it as a development material, a gift item, an event item, or have any status effects attached to it. You can sell it to any store and receive a sizeable amount of Zeni.
To farm these types of items, you want to explore the world and hunt down any creature you encounter. You can find dinosaurs, fish, deer, gazelle, wolves, and many more. You won’t always get items like these off of them, but if you capture enough of them, over time, you’ll find things to sell at a shop. You should also check out the gifts section because some of the items lower down on the list do not enhance any of the soul emblems. Instead, they’re high-quality items you can make a great profit off of in the game.
Your best bet is to complete any of the side story missions you encounter and hunt down any animals you find. The more you do it, the more Zeni you’re going to have in your pocket.