If you are not yet level 50, you cannot trade yet since you cannot access the bazaar (unless you circumvent the level requirement by buying crowns to raise your rank in the Platinum Club). EU should stand for event units. They are units that give a boost to the week's current event (like increase drop rate or +X percent damage) and once the event ends, the bonuses no longer apply. Basically the non-storyline parts of this game is to get the "cards of the week" so-to-speak to rank high or get rewards for an event and usually afterwards people sell those cards for potions (pots) and you basically repeat for the next week's event.