There's no magic shop at my table either. The Great Market I referenced consists of thousands of individual dealers in various sorts of goods (everything from basic commodities, to futures, to priceless treasures) and the even larger network of middlemen and agents who try to connect buyers and sellers.That's about the same as how it goes here.
This is something else I specifically try to avoid: bidding wars. Why? Because it breaks the single-value mold, and if nothing else makes treasury division an absolute beast in that every time the bid goes up, everyone's share value changes.
I've always kind of seen it that the greatest demand for most magic items is likely to come from adventurers, be they PC or NPC, or nobility; meaning most transactions are likely either going to be a) private deals with contacts you've made during training or through guilds or wherever or b) direct sales to the crown or a temple or some other high mucky-muck. Some class-based guilds (e.g. a mercenaries' guild, or a wizards' guild) might act as clearinghouses and-or contact points, and occasionally buy or sell things as a guild, but that's it.
There's no "magic shop" as such.