Note: Most of these costumes will auto-consume when you buy them, meaning they do not appear as an item in your bags which you can then sell back to the vendor like you can in other areas of the rift store.
Some of the costumes are required for cheevos, you will receive the actual item for these, to credit the cheevo you need to equip the item in your gear screen (not just your wardrobe screen) this will make the item bound to account so you still can’t sell it back!

I recommend you farm the currency for costume skins on alts and use those alts to buy the skins. All wardrobe unlocks are region wide.
I also recommend you WAIT til towards the end of the event before you buy your costume items, this is because you may get many of them as random drops from minion adventures or from other in game activities.