I typically have a teleporter located close to the entrance of the plot, that is set to friends or higher access, that 'ports to an otherwise inaccessible area of the house. I've had even very tiny houses with vendors that were reasonably secure this way.
Here is one of the smaller vendor houses I designed. I successfully took refuge from PKs in the tiny thatched shed up top, via a teleporter under the mat.
The house was just a holiday novelty one so it was very basic, and only up for a few weeks. I wasn't too concerned about security while stocking, but if I were, I would have made half doors that I could set to owner only while doing so, as in this design:
The roof was also designed to be able to place vendors around, with teleporter access that could be restricted.
This house I used for a community center had greater need for security. I used the vestibule area for security. I'd let someone in the outer doors, and close them behind, then I'd take the teleporter on the porch inside, run detect hidden, then open the interior doors. (The doors were left anyone access unless I was decorating or stocking).
As mentioned, there are lots of ways to make your house secure. I don't think anyone ever successfully killed me in any of my houses, and there were more than a few attempts.
You basically want to account for physical proximity of thieves and PKers when you are stocking the vendors, and the annoyance of field/area/range spells that might reach you even if you are out of melee range. Since your customers can reach vendors via the house sign and double clicking name plates or the actual vendor, their ability to directly reach the vendor is less important than the vendor owners.
-Skylark
