This page of the Biomutant game guide explains how to accumulate Green, the local currency used in all transactions. You will learn the best ways to get rich in the game and what to spend your cash on.
The currency in the game is known as Green. You can check how much cash you currently have in the inventory window - it is displayed in the upper right corner.
The main way to acquire Green is by selling items to merchants. This way you can dispose of unwanted weapons and equipment, excess consumable items, or objects with no additional use.
It is worth noting that the prices at merchants are affected by your Charisma attribute. The higher your Charisma, the better prices you will be able to get for the goods you sell and the lower the cost of the items sold by the merchants.
An alternative way to earn cash is to find Greenleaf Plants - an example is pictured in the attached image. Jumping through the "hoop" gives you a one-time cash reward, earning you a a few hundred Green. Take advantage of this opportunity whenever you come across it.
You shouldn't be in a hurry to spend your Green. Weapons, equipment and consumable items can be easily obtained by carefully exploring the area and completing the locations you discover. It's not worth spending money to buy these items from merchants.
On the other hand, Green is worth spending on unique objects that would be difficult or impossible to find (e.g. the certificate pictured in image no. 1). It might also be a good idea to buy difficult to obtain crafting components in order to use them to upgrade or craft powerful weapons and equipment.