You can buy items for your character, and sell the unnecessary things here.
You can choose the desired items from a pull-down menu,
then you can specify the number of the items to be bought.
Selling items works the same, but you can only sell items from your backpack.
(That's why there is the Put worn items into your backpack icon on the shop-screen:
If you push it, every item on your body will be put into your backpack.)
The shopkeeper buys your items for half of their retail price.
If you want to have information about an item, push the ? icon after selecting it from the pull-down menu.