Any store where you can purchase items will buy similar items. To sell items, when you are in a store inventory window, simply click on the Player button and the inventory will change from the store’s inventory to the items your character has in his possession.
Select an item by clicking on it for a description of the item, and the price the merchant is willing to pay for it. If an item is listed in red, it can’t be sold. If an item is in use by your character, then it won’t be listed in your inventory to be sold.