that's partly wrong tho, and kind of common misconception of "player-driven" economy. the owner/developer/publisher/administrator of the game also act as "regulator" or "government" body of the world/game. so, yes, you can do, or "fix", something, and in fact, you're the only one who can "flip" the economy upside down if you want to.
i sometimes, discuss mmo's economy with a friend, and the basic problem on every mmo economy are:
unlimited "reserve" of in-game currency.
ineffective or sometimes, non-existent, sink for the currency.
these 2 problems, should be solved within the initial game design (i.e. on analysis, design, or development stage of the game), so, we can't do much about it.
if we back into celestia luna case, my realistic possible solutions would be: (i'll say it blatantly, and what i said might be a little too radical, or in-accurate because my knowledge about the current state of the game is not much)
stop "printing" additional "money" (in-game currency) through private donations (if you're still doing it).
i understand that private donations are the source of income for you guys. but, you should start "regulate" or limiting yourself first if you wanna help controlling the inflation. the world/game already has unlimited reserve of money waiting to be "mined" or "farmed" by the players. by "printing" and giving it to a player in return for private donations, you only add more "money" to the market, and it's only make the inflation worse.
start designing and implementing more sink(s) for the in-game currency.
you should start gathering data about "how much money currently 'exist' and owned by the players in the game". then set a goal on how much money you want to remove from the market, and finally, build a plan or strategy on how you'll do it.
for starter, you could start selling more "premium" items directly from npc, that players can buy with in-game currency, and make those items account or character bound. so more money can go into your sink, and prevent those items become "trade goods" in the market (you want a sink, right, not a flow for the money to move in the market). you can make it either permanent or temporary as a long duration event (like a month, or 3 months, etc). if you make it temporary, some players might start "rushing" for it, and some don't. if you make it permanent, it would another sink for the currency.
the game already had some sinks, like crafting cost, etc. you could play around it, like increase the cost for the end game stuffs, etc.
gradually decrease the amount of in-game currency that a player can "farm" at a time.
there're some possible solution for this. you can decrease the server drop rate multiplier. or, you can remove the in-game currency drop from monster. or, you can decrease the price of specific item that being used as source of income. or, you can remove the server drop rate multiplier completely and only increase the drop rate of some selected items (raw crafting materials, trash items for money income, etc), or, you can combine those proposed solution from me, etc.
basically, currency sinks in game (in this case, mmo) design, is the practical implementation of fiscal policy for the game economy. it can be in form of taxes, maintenance cost for equipment, production cost for crafting, housing rent cost, etc.
in mmo, if you can design a good sink, then you create a good policy for your game economy. if you're doing a good job adapting your game's currency sinks to the "evolution" of your game economy, then you're doing a good job as the "government body" or "regulator" for your game.
and sadly, almost in every private server version of an mmo like this, the server wide drop rates multiplier often times become one of the factor that driving the inflation. given how "easy" it is to generate income (in-game currency or money), it's only matter of time and the prices will skyrocketed naturally, regardless the supply and demand chain. also because a market weren't only dictated by supply and demand law, it's also dictated by the amount of "money" circulated within the market or (active) population.
uh, somehow, it become an essay. lmao.
well, i like the discussion about mmo economy, so, can't really help it.
best regards, and ciao...