Hi, i think i understand the second question:
Well, after buying crystals, you can:
a) use them directly in the boutique (Y) to buy the items available there or
b) convert them to in game gold via the exchange. That way you can buy items from other players, whether they are boutique items or other game items.
Note that not all items, or 'currencies' in your (I) tab are directly buyable in any way.
You need to earn some of those through in game activities as they are bound to either a) your character or b) your character and its incarnations.
All the items that are or were at any point available in the boutique are bound to your character and its incarnations after their use.
So you need to buy those only once if they are permanent.
About using premium crystals: as redcatcz said, the items bought with those are directly bound upon purchase, not use.
So if you have crystals, NEVER convert them to premium crystals, unless that is the required currency to buy a specific item in the boutique, and then again only convert exactly the amount required for that purchase.