I think the issue here is that these communities (regardless of size) may have a limit of (eg) 50,000gp for magic items, but due to the sheer number of items, might have several million gold pieces worth of stock. Or the counter side, have someone available that knows of and is able to create any magic item.
If this was the case in my campaign, the shop would be robbed (or attempted), and that *ahem* magical NPC with the magic item feats would be captured and put to work under armed guard.
The rules for community wealth means that no "magic shops" exit. Their isn't one store for the players to walk into and browse [Or case]. The rules on it state that using community wealth guidelines is abstracting the search for the items. The players may find the belt of strength they are looking for in the back of a curio shop while another PC finds some scrolls to buy off the old wizard done the block. WIth the MIC adding that if you need an item extremely quickly that Gather Info checks would be needed.
Since you have no store, you have no stock. Sure if the players are looking for a specific item they could rob the person who has it, but in order to get the stock of items in the town they'd need to rob the whole town of both magical and mundane items.
Which is far beyond being able to rob a single store, and if your players are robbing cities more power to them.
The concept for me, or I should say my players, lends itself to a range of complications. I cannot say "for arguments' sake, every item is available, but you cannot steal it"; it detracts from the gameplay and feels like I'm breaking the rules for the benefit of the players.
Well for arguments sake say the items you're looking for are available rather than any item is available. The difference is that the players get the items they are looking for, and the characters don't need to know the town had any other options.
I don't know why it feels like breaking the rules [assuming 3.5 ruleset] because the DMG rules support the idea that any item under the community wealth be available [with the caveat to make certain exceptions for times of rapid growth or extreme decline]. Though the MIC adds the further caveat that if want you can make an adventure about finding a particular magic item. if it with further the story to do so.
Now 2E and before I completely agree. The game was set up far differently when it came to players gaining magic items. The game was set up that wealth was the main reward for XP, where as in 3.5 gaining wealth was just a way to be able to compete against increasingly powerful enemies.
All my shopkeepers have a limited number of goods and are of a certain level related to their goods (the protection of those goods). They have guards, and traps and magical wards, etc to prevent theft. The players know this and accept that there's a risk in attempting to steal what's available.
You could argue that I am able to do a similar thing for the 'all items' option, but then I'm further complicating the system to prevent the players breaking the game; so they don't get everything they want - or is that not a bad thing?
Well if you want to detail every shop to such a degree than more power to you. I haven't run across a 3.0/3.5 group that has thought they could rob a magic shop [though they did attempt to rob a bank, but that was in the adventure].
As for being a bad thing, I don't play in your game so I wouldn't be able to judge. Personally for me it would be a bad thing because I see no reason to house rule away the DMG rules on players finding items in towns. To me that
feels like I'd be breaking the rules to benefit the GM and spite the players.
As a curiosity; how do your players know they aren't meant to or cannot steal from the shops or kidnap the item creator. Is it a verbal agreement out of game or something present in-game?
Well they did that stuff all the time in 2E and 1E but they never even brought it up in 3.0/3.5
I think that they understood that the items aren't coming from one shop but some could be coming from and old widow who's husband died from poison after an adventure and has been selling off his meager surplies to survive while another players +1 mithral chainshirt could have been found in the stores of the local barracks and the commander maybe willing to sell it to feed his own pockets.
And while both options could be expanded into adventures or stories in most cases it would be a simple side track that would just get in the way of the main plot(s) of the campaign.
Though it would work well in a more sandbox game.