What if it wasn't a currency but the weapon gained experience as you used it. It would make exploitations to acquire it impossible (at least any that I can think of). Maybe it could be special exp gained only in dungeons or something like that to make the exp more rare. And to make sure no one starts making a lot to sell, they should be bound to the character. Of course this should start after some epic quest to get it, and maybe to get the final form you have to fight a really hard boss. Possibly one on one, with the boss set to each class (so that healing and support classes don't just fall over in five seconds) so that it is a true test of that player's skill.