For item and player issues we typically suggest sending a ticket in via Customer Support, however this particular instance is unique.

In regards to real life currency sales: If the player is suspected of trading for real money, this is against our Terms of Service. Please report the player via SUPPORT at the top of the page. There will be an option to "Report Player" when creating a ticket. Please use @ (name) for a game name and Gamertag (name) if you have a gamertag. Let them know that he is suspected for gold selling for real money.

In regards to guild losses: Support unfortunately cannot resolve matters of guild theft or politics. Permission is granted by a guild leader to guild members to use guild banks and thus this is at their own risk and discretion. It is still worth mentioning in your report in the event that he does this frequently, however typically there is no resolution for these matters.