You know, things that can be sold (either to an NPC or the Marketboard) but can only be actually used once, like Minions, Triple Triad Cards, Orchestrion Rolls and such.

When you get those in dungeon/raid chests and you have already used them, they should become Greed-only, in order for you to still be able to get and sell them if you want, but without harming those who don't have the item yet. The preference is their, and if everyone has it or don't want it at all, feel free to greed and get it and sell.

Of course, it opens the possibility to a practice of "I'm never gonna use this item in order to always Need and sell", but I believe the amount of people doing this would be pretty low, enough to not make this a problem.