There is a function built in to Anarchy Online that "mutes you" temporarily if you send messages too rapidly. This is probably based on monitoring how quickly messages are sent from the client, or maybe recieved at the server (though I doubt the latter).
How about expanding on that function a bit, so it actually checks the contents of the messages, and "flags" the ones that are suspected to be about credit selling (or other "illegal" activities)?
It wouldn't be a huge annoyance for everyone else (all us non-credit-sellers) if it was done in a somewhat polite manner. If you made it client-based, you could even make it a feature you can turn on and off in the options! That way if you're scared you'll be missing out, you can just turn the filter off.
An example:
You just wrote a message that got flagged. Apart from seeing the message appear in the channel you get a message like the one you get when you're "suspected of spamming";
"Your message has been flagged as suspected of violating the terms of service. Players with the chat filter turned on will not be able to see it."
I'm not saying that would eliminate the problem, but you could probably catch a very large portion of the credit seller messages. Rather than spending your time banning all these accounts, why not just take their voice away?
Of course you would need to allocate a few hours of man-power to maintain the fuction every now and then (when the credit sellers find ways of circumventing it), but it shouldn't break the bank on neither man-hours nor server resources.