Gems are the in-game currency and can be used to purchase items from the Item Shop.
Premium items can be purchased from the Premium Shop, and may help improve gameplay.
The lucky boxes provide players with access to top planes, equipment, and benefits that will help them give the edge in combat. Each option provides the players with 5 possible boxes to choose from; depending on which one they choose the player will receive a random item(s). Furthermore, players are also provided with slightly more expensive Lucky Boxes that have increased chances of getting the best possible outcomes.
The Black Market provides an alternative to grinding mission after mission. Players can spend hard-earned Gems to purchase random items from the designated level ranges. This allows even casual players to have a chance at the rare gear that helps change the tide of battle. But don't think of this as an easy mode; players may also receive worthless items, and the black market should never be completely trusted.
You can sell your items in the square and price them at your will.