"Greedy players" isn't a player problem -- it's a Game Master problem, mostly [edited based on the sad face reply...]. ALL players want good powerful stuff, even if they're role-playing a poor, homeless unsung hero of altruistic demeanor. It's the GMs job to tailor each reward/magic item "found"/"won" they put in the game to each player, where everyone feels rewarded and no one feels left out. Certain items could have "side-effects" if used by another character class, say, or some other inventive GM strategy.
Hope this helps!
