MMOs are moving towards a single-server model. But this brings up an interesting wrinkle with Auction Houses or markets.
Markets with a very large number of participants are hyper-efficient. As Diablo 3 and Guild Wars 2 proved, they are so efficient that they cease to be fun. Margins get rapidly driven towards zero. While that's great for buyers, it's not so much fun for sellers or traders. And we're all sellers at some point, if only to sell a neat Bind-On-Equip item we found while leveling.
I think what the last few years in MMOs have shown is that markets need to be somewhat inefficient to be fun. They can't be too inefficient, as in the case of too small servers, because there needs to be enough supply, and somewhat predictable prices.
It is interesting to see how the current single-server MMOs are handling this issue. The grand-daddy of this model is Eve Online. Eve has many markets. Every solar system is a separate market. These markets are separated by distance and time. This setup is pretty good. Some systems (Jita, for example) get known as trade hubs. Moving goods between trade hubs provides for arbitrage and interesting gameplay for the goblins among us. It's also a system which feels very natural and realistic.
However, this setup requires that your universe be set up in a certain way, with significant travel time between each hub, and a population spread out across the universe. It is not really a model for themepark games where people tend to follow the same "flow" as they move from zone to zone.
The Elder Scrolls Online handles things slightly differently. Each guild has its own auction house. Each guild is capped at 500 members. But a player can belong to up to 5 guilds at the same time. So most players will end up joining two or three "trade" guilds, which are dedicated to trading. This does give a decent selection, and also allows for a bit of arbitrage between guilds.
The downsides here are that it does seem a little weird to have these continent-spanning guilds which cannot talk to each other. As well, most guilds will want their 500 players to be active. I'm not sure how someone who plays very casually, maybe logging in once or twice a week, would fit into these guilds. Too many of these types of players, and your guild market becomes dead.
All in all, this is an intriguing problem. As more and more MMOs move towards the single-server model, it will be interesting to see what new solutions are brought forward.