Rupees are the main currency used in The Legend of Zelda Breath of the Wild. Most shops and characters take these gems in exchange for their services or items. Unfortunately, the only items that can be sold to shops in Breath of the Wild are armor, materials, and food. The quickest way to earn Rupees is to start cooking. See the How to Cook section for information on how to cook meals and create elixirs. The objective is to use materials with the same effects to make high quality food that can be sold for a high price.
When it comes to selling Materials, these can be sold and replenished most of the time, though the tougher monsters don’t drop loot as often. It’s fine to sell 50 Bokoblin Guts when you have 100 of them. However, it’s always best to make food and elixirs first, use the material to upgrade any items second, and then sell the materials for currency last. Monster parts and natural materials are used to make food, ancient materials are used to upgrade the Sheikah Slate and create ancient weapons and armor, and minerals are used to upgrade armor and jewelry.
While exploring Hyrule, you might come across glowing, blue rabbit-owl hybrid creatures. These are called Blupees, and are often found near Great Fairy Fountains. Blupees cannot be killed like other creatures in the world. Attack them from a distance with a bow and arrow, and they’ll drop the Rupees they’ve collected before disappearing.