There could probably be an NPC exchange surprise event for 1 day, or similar, instead of putting it directly into the mall, so it needs hard work in the process. But then again the most prepared people would be the ones from mall, in planned and unexpected event types.
Or just IT'S A BETTER IDEA NOT TO DO IT EVER AT ALL to make the server last longer with the lower gem quantity absorption of the playerbase. We could even view the matter as there is a certain number of UG colos, rages, souls, azraels (?!), etc that any server can handle before everyone's power is equilibrated and everyone might as well have no gem to keep the same kill ratio, so the longer it takes the better. But not too long. The more unfair advantages the server has towards someone new, the better for longevity but worse for new player. (because the older and newer players see that time pays off).
Wait. Are hardcore mmorpg players all masochists then?
I still have not seen even a single Unique Gem in the market directly, only seen spontaneous and random tradings in world chat. They are a rarity. Are they 12M gold?