If one of your characters has an item that another needs, you can hit the 'Trade Items' button from the 'View Character' screen (of the character that currently has the item(s)) to transfer them. From the 'Trade Items' screen, simply select the item(s) you wish to trade, then the character to give them to, and hit 'OK'. Note that the character receiving the items must have enough free inventory slots to carry the items; if not, the trade will not take place, and an error message will be displayed. Also note that a character may not trade items currently being wielded or worn. To trade these items, you must first take them off (by hitting the 'Equip' button and proceeding from there).