You can use the marketplace to sell items to other players. Click an item in your inventory and then the stock market icon to bring up the marketplace form. There you can put the quantity and # of items you want to sell. There is also a deposit that you will have to pay to put the item on the marketplace shown as red text on the bottom. The deposit depends on the value of the object.
Once put on the marketplace, it will show up in marketplace searches for 1 day (2-days with a club-perk).
Buying items from the marketplace is just like buying from the store.