I've read around that it is impossible to steal from Geoff, but actually I've been able to!
I always leave one pyramid at the Ars Magicana teleport because it's near the merchants. Sometimes, either coming back on the teleport or on the pyramid, I walk down near Geoff and the door to his place is open! I walk right in, Geoff still at his anvil outside the house. I picked up all the weapons, checked out the back rooms, and eventually sold them to Geoff. I don't know how or why the door is open, but it happens often. Is this a peculiar glitch to my game or does it happen to other people too?