What about possibly bringing event items back each holiday, but they can only be purchased on the mogstation until the next time the holiday comes around? In addition, make the event items unsellable on the mb (future ones, not ones currently up for sale) and if the person that purchased the item is no longer joined to the estate the furniture is used in, it's placed on the local storage npc (like what happens with furniture when a house is demolished). This would mean the person using the furniture would either need to purchase from the mogstation, be gifted the mogstation code or actually be around for the event. For most of the clothing items trading isn't really an issue since most of those have always been untradable. Being that housing has been such a mess with this game, I don't think it's unreasonable that a lot of people never picked up some of the event furniture figuring they had no chance at a house and thus no use for storing the furniture.
In addition how about after a decently large portion of time (1/2 - 1 year) mogstation exclusives stay on the mogstation but are also obtainable via gameplay in appropriate areas. Maybe there's a low drop chance for the manderville outfit for example in all of the manderville related trials. Maybe the moogle mount has a low drop chance from mog ex or even an obscene amount of currency from the moogle beast tribe? This gives players that don't want to face a long grind or rng the opportunity to still purchase it and allows people to purchase it earlier than the in-game obtainment availability as added value, while still giving people the chance to earn these items by playing the game.