Click on an item with the left mouse button. The item will be highlighted, and the purchase and sale prices will be indicated at the bottom of the screen. These two prices will not be the same since the shop buys at a lower price than it sells out (the shopkeeper is making a hefty profit!). You can then choose to buy or sell the item by clicking on the relevant buttons.
The price of items in the shop also depends on the current stock. If a shop has 500 swords in store it will sell them at a reduced rate to try and clear stock, however it will also pay less for them if you are selling.
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