For the armoury, the merchant wanders around a bit, and sometimes turns his back on you, leaving brief opportunities to steal (save first). Also, if you place a container in your inventory, it is not considered stealing to drag and drop items into it (it is stealing when removing the items from the container, so make sure you are alone before doing so).
For the potion merchant, the first time you open the door he will close it. If you Lockpick and open the door again without trying to enter (without moving?), you can toss a teleporter stone through the door and close it. See the screenshots and description on
Kiya's site (that and the next page).