Why would they sell magic if no one else in the world could use it? it's not like they know the light warriors are coming:O

And I remember my reasoning behind the one-use idea. In ff2j (which admitedly isn't ff5) you can either give a person magic to use or equip them with it as an item, which will cast it at like level8 once.

Plus it sounds sorta realistic, when you see all those google ads of people buying a spell over the intarwebz xD