When you want to sell some items or if you want to buy some new equipment, ask an NPC for a trade. The NPC Trade window will open up. By pressing either Sell or Buy, you can quickly see which items you can sell to or buy from that NPC. If you select an item, also its graphic will be displayed. By clicking on the graphic, you will gain information on this item (flavortext, attributes and weight). The price is quoted behind an item. By using the slide bar, you can select how many of an item you would like to buy or sell. The selected amount, the price for the selected number of items and the money you have in your inventory are shown below the slide bar. Finally, press Ok to confirm the trade.
If you do not have enough money, space or capacity, a warning message will be displayed and the trade fails. Simply lower the amount of items on the slide bar until you meet all conditions to complete the trade.