Incidentally, I might add that your own MUD and virtually every MUD on earth falls in this category of "optional payments may influence gameplay."
If the concern is actually "introducing an OOC variable" (nevermind that every MUD is already full of OOC variables), then it doesn't matter who the money is going to: It's precisely as OOC whether you're paying real money to another player or to the game admin for that Sword of Ubercoolness, and there is no MUD I'm aware of that can make an honest claim that there is no way a payment of real money could ever influence gameplay.
--matt
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