If you do not have the level required, you still have a chance to purchase the item using the early unlock. You can pay extra Gold to unlock the item and then you can purchase it, without having to reach the required level.
The unlocking cost gets every level higher by 5 gold, means if you are level 1 and want to equip a level 5 item you will need to pay a 20 Gold fee to unlock them.
(Desired Item Level - Current Level) * 5 = Gold Fee to early unlock
You will know if new items are available in the Shop when a red exclamation mark is shown in the left corner of the Shop Icon. This is especially useful during a series of matches so you won't need to check everytime the shop if there is something new.