Some more specifics for you: He's done this only 2 or 3 times, but has made off with large amounts of junk, which he then sells in a -different city- (Namely, Greyhawk vs Enstad). Yes, yes, I know all of you just said "Greyhawk?! He's trying to screw over the thieves' guild in -Greyhawk-?!" I'm going to get right on that, don't worry. Anyone got some specifics on that guild?

He also basically breaks in a window, at night, into the storeroom, then teleports out. (Wand of teleport). I know there's all sorts of fun spells to impair teleport, but I'm trying to think how many merchants would actually have them?
Either way, he's probably going to have the Greyhawk TG on his butt very soon. Right after he saves the world (next session).
Thanks
Eltern