If I purchase NCoin on my game accounts can I use it to buy items on either?

Yes, as long as your Lineage II, Blade & Soul, and your Aion* game accounts are associated with the same NC Account. You can use NCoin freely across Aion*, Lineage II and Blade & Soul from the same wallet.

*Aion no longer uses NCoin so any purchased NCoin for Aion will need to be converted to Black Cloud Coin.