Fortifying & Defending a shop in a city

When they’re not adventuring, the PCs in my campaign own a magic item shop in a very large city. They have a staff, inventory, marketing, and in general it operates like you’d expect an item shop to operate (except a lot of their inventory is originally dungeon loot). This city has a criminal element. That means that theft is a risk. At low level their inventory was mostly mundane equipment and the occasional minor magic item, so security was limited to a guard and an alarm spell activated when the shop closes for the night, to alert the two PCs that were sleeping on the top floor.

They’ve outgrown that now, as they’re level 8 and the quality of their loot (and inventory) has increased considerably. A low level fighter type is not an adequate guard against high level magic thieves, and with tens of thousands of gp of items in the shop, it’s a tempting target. (Also the PCs themselves aren’t always there and can now afford apartments, so they don’t sleep there even when they’re in town.)

I’m trying to figure out how a building like this would be defended in a D&D city. I’m interested specifically in night time when the shop is closed, as during the day it’s in an area that’s well patrolled by the city guard and heavily trafficked, so a full on raid is highly unlikely. Shoplifting can occur, but the staff are fairly keen-eyed so it’s rare.

Some specific requirements:

  1. The building already exists, and is not designed to be fortified. It’s a good building with two floors (and a basement) and is masonry walls & wood construction inside. The city government is not going to let them demolish it and put up a fortress, and for story reasons they have to stay in that location. (The main floor is their show-room, the top floor is private, and the basement is used to hold inventory and some other things behind a hidden secret door.)
  2. The building has to be inviting when the shop is open. The PCs income entirely depends on this, as their loot is sold in this store. That means it can’t have disintegration traps that go off when someone’s kid wanders into the basement. 🙂 In general, magical defenses must be able to be deactivated during business hours.
  3. As the shop sells magic items, people want to be able to inspect the merchandise. That means the building can’t be covered in an antimagic field while the shop is open.

Specifically, the PCs are concerned about theft. Primarily of the "sneak in at night and rob the place blind" variety, as during the day the store is occupied and the city guard have frequent patrols in the area (trying to barge in and take stuff by force would not end well for the attackers unless they’re incredibly powerful). Stopping shoplifting beyond that in a way that doesn’t cause the customer problems is another issue, and they’re not overly concerned about it (plus it’s hard to shoplift full plate without being noticed).

The criminal element in the city ranges from petty thieves & con artists to a full blown organized crime operation. That operation has access to spellcasters. It would tax their resources to do something like teleport people into the building directly at night, but it’s not beyond their means.

Suggestions that use D&D 3.5 source material would be ideal.