If you obtain equipment you don't need, or if you're having trouble getting a particular item, you can either sell or buy items through the Exchange.
When you tap the [Exchange] menu from the main menu (≡), you'll see a Buy and Sell button for the Exchange.
Your character needs to be at least Lv. 30 to use the Exchange. Keep in mind that Exchange Fees will be deducted from items you sell at the Exchange.
Items you buy or Gold you obtain from the Exchange will be sent to your Mail, so make sure you take them out of your Mail before the expiration period!
Also, please note that you can use the Expand Slot button to increase the number of items you can list in the Sell tab.