You can enter the shop far enough to see them, then swap places to get them to the door, and repeat the process once or twice to get them outside and out of the way.
There can sometimes be a small gap in the north west corner when two walls meet, which you may be able to see through if you stand at just the right place. If a gap exists, though, it may still be blocked by furniture, etc, in the inside corner. For example, there is such a gap in the mage Kistandalius shop in Ars Magicana, though it will not help to get him out of the shop; he will teleport you away from his back room whether he can see you or not.
There is an easier method to steal, though it is a bit of an exploit.
If you open a container inside your inventory and drag and drop items into the window it is not considered stealing. It is counted as stealing when you remove the items from the container, though, so make sure there are no NPCs around to see you when you do.
A potential problem doing this is described below;
weight bug and disappearing chests (broken links are fixed in a reply, and the screenshot links re-fixed in another)
