You drag the item into the box, set the price you want to sell it for then click "Sell Item".
You might be trying to sell thing for more than 60fg (which you don't currently have the fg to do), there is a 5% selling fee, if you sell an item for 19.99fg or less the fee is taken out of the fg you earned from the item. If you sell the item for 20fg or more, you must pay the fee prior to placing the item on the market.