We will show you how to use the merchant NPCs. There are 4 types of merchant NPCs, Consumable supplies merchant, Weapons merchant, Armor merchant and Accessory merchant.
Each NPCs can be used by double clicking on the NPC with left button or click on the NPC with right button to bring out the menu and click Trade.

Once the store has opened, double click on an item or click purchase to purchase an item.
It will ask you one more time in a separate window and if you click on YES, the item will Automatically be placed in your bag and certain amount of game-money will be consumed.

If you want to trade in your item for game-money, double click on an item you like sell or drag an item from the bag into the store window.
Consumable supplies merchants are located in each territory whereas Weapons, Armors and Accessory merchants are located only in the starting village and the Capital Castle.