Some items in the store have to be unlocked before you can buy/use them. These items include:
Golden Fox Balloon
The Golden Fox Balloon can only be obtained by earning 1st place in Elite League when you already own the Golden Fox. You have to be in 1st when Leagues reset on Wednesday to win the 1st prize.
Each avatar has one master skin that can be unlocked by winning 300 games with the avatar. You can see your progress towards unlocking it under each master skin in the store: