in game
quite ez acutally, all they do is make a word filter in game... so cause according the user agreement they can monitor any chat, so with the word filter in place say "fg, jsp, d2jsp" etc etc if you say anything like that your logs would be flagged and then its just a simple matter of reading the logs (Which include whispers etc etc) to verify a sale has occurred then its just a click of the mouse and your banned.
as for the using multiple accounts.. they are monitoring IP;s with the logs.. do you think it would be hard to just ban all the accounts that have connected with the same ip...

they are monitor you in game as well...
clearly alot of you dont know alot of about computer game programing
Then... We have to counter that by ALWAYS saying d2jsp, bilions of times so searching the logs becomes pointless!