In the store under the Events tab the guardian games armor sets are available for both silver and bright dust. When trying to buy any bright dust set it says “Not Enough Space”. Not sure what that’s about.
Answered, thank you
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Anytime that's happened to me it's because they get put in the inventory as unlockable cosmetics, and it was full at the time. If you haven't been "unlocking" them, your inventory could be full. Also, it could be a bug. There are so many bugs.