As you approach a merchant, the action button on the bottom right of the screen will change to indicate that you can open a trade window.
In the window you’ll see your inventory on the left and the merchant’s on the right.
Select an object to show its value.
You can choose multiples of the same object by clicking the button .
Click on the buy/sell button to accept a transaction.
Note: the selling price is much lower than the buying price in Yslandia… a strange tradition, but so it goes, so consider carefully before buying anything.