I wrote an article for the IGDA Newsletter trying to better address the security concerns in MMO trade systems I discussed earlier this year; even if you read the previous blog post, I think the new one is worth reading by itself. Thanks to Scott Jennings, Peter Harkins, Ammon Lauritzen, Jeramey Crawford, and John Henderson for feedback on earlier drafts. I wasn't able to incorporate all of their feedback, but the article is better for everything I did include.
EDIT: boy is my face red, I did not include John Henderson. My apologies.