@Leek Yeah, the market should be oversaturated with these. We were shown that imperial swords are rare, but there are still quite a few around when decently wealthy people can afford to collect them. This would drive the price of crystals down. Unless each has a different signature crystal. Meaning that most of the other imperials don't poop the same ones like these 3, so the price of these would remain unaffected.
They only shit crystal when they have too much mana, more than they can normally release.
This happens when they eat A LOT, and no all imperials sword eat food.