If you have purchased an item using the in-game mall, you can immediately open the “Item Bank,” located on the menu bar, or press “alt-I”.

A window will pop up asking if you want to open a cash item bank. Press the circle and the item bank will open.

From there, move your item from the item bank window to the cash item window.

Some items can be used or equipped directly from the cash item window, but some require that you open your inventory (the “item” icon at the bottom left of your Seal Online window, or press “I”), then click on the “cash” button at the top right of the inventory. Items such as Golden Chest Keys can be used from this window.