This merchant ‘district’ can be found in any reasonably magical fantasy setting; it typically operates inside a single large building. The more decrepit, the better: the wooden beams should be scarred and worm-ridden, the metal nails must be staining the walls black, and the roof — well, nobody in the Grumbles ever actually sees the roof from the inside, but it should be about to fall apart, or possibly over. Oh, and the building should be on the local riverfront. Easier to get shipments in and out that way.
As might be deduced from the above, the Grumbles is mobile; buildings in multiple cities will host a version of it from time to time, in a reliable but not regular pattern. Inside, the site is made up of a series of individual rooms, each crammed with goods of all kinds, types, and certainly prices. There is no main common area, and indeed not even any corridors; people get from one vendor to another by going through each room, one at a time. And the rooms shift their relative positions between visits. When they’re not shifting during visits.
It’s a lot less confusing than it sounds, because the Grumbles responds to the intent of its visitors. If someone is visiting to find, say, a magical reagent, and it’s available for sale somewhere in the site, that shopper will come across the right vendor within ten minutes or so of searching. If someone is looking for food, there are plenty of people selling that, and there’s at least one tavern located deep inside the Grumbles. And if somebody doesn’t know what they’re looking for, the further in they go, the more interesting the goods become. Many people like to browse a bit before they buy.
Note that the vendors themselves are not magical; they simply have the ability to infallibly find their own rooms, quickly and conveniently. Getting new stock is their affair, but adventurers often happily trade items for items, and of course things can be delivered to the Grumbles as well. Vendors don’t pay rent, but there’s something that weeds out unsuitable vendors (‘unsuitable’ being a fluid term, of course). One needs to have a flexible mind.
Do local authorities hate the Grumbles? It depends. Having an unaccountable magical location in the area tends to make the Watch nervous; on the other hand, the Grumbles is a reliable if not regular source of otherwise unobtainable items and artifacts. Most take the position of watchful neutrality, but any scheme involving the Grumbles will swiftly gain official notice.
Lastly: do not steal from a vendor in the Grumbles. It’s widely believed that anybody who does and then leaves the room will never enter another room in the Grumbles, or indeed anywhere else. This belief is bolstered by the way that the stolen goods invariably reappear in their original room, in short order. Nobody knows how it happens; and the vendors could not care less, either.