Firstly you need the item you want to sell in your inventory. Then once you see the red-stained glass, just click that item. In the new GUI click the lime glass pane and type in chat however many $ you'd like to sell your item for! Then click the yellow glass pane and type in chat how many of the item you'd like to sell per the amount to chose to sell it for. If you'd like to buy that item from players select the red glass pane and chose a price you'd like to buy said item for, note we don't recommend this but the feature is there if you need!