Buying in-game items or money from other players, using real money?

[notyourbroom]

I assume that this is not allowed, but I triple-checked the rules and ran a couple of searches in this forum and can't find any prohibition against it. The closest thing I can think of would be that it might be considered playing "out of roleplay" if someone asks for, say, $5 US dollars instead of $20,000,000 credits for a certain item or weapon when offering it for sale in system chat. But on the other hand, since OORP chat is not forbidden in private conversations, I don't see how it could be illegal if it's negotiated in a private channel. If a newbie to the server privately offers a veteran player real-life money for in-game credits, is that really punishable?

Has this ever come up in the past? Like I said, I did some searching, but I didn't find anything.