Now that I've said all this about markets, let me make a few broad presuppositions about typical players. For the most part, when players engage in the market they want these things:
Final Fantasy XIV's market system is based on the "bazaar" system seen in many Asian MMORPGs, including its predecessor, Final Fantasy XI. The bazaar allows players to place a number of items (in FFXIV, 10) for sale at a listed price.
Once created, players with a bazaar bear an icon next to their name indicating the presence of a bazaar. Other players can click on that player and chose "browse" from the list of player interactions (along with trade, invite to party, and so forth). This opens a window just like an NPC store with the items for sale and their price.
The purchasing player conducts the transaction just as they would with an NPC merchant: selecting items and quantity and, if they have enough money for the transaction, they get the items and the bazaar-bearing player gets their coin (minus a transaction fee, of course).
Square Enix has declared that it wanted to rely on this system because it encourages "creative selling" and makes players feel personally engaged in market transactions. This is all ridiculous head-in-clouds dreaming, of course.
The actual reality is to find areas crowded with AFK players all sitting around with their bazaar icons up, and other players browsing the crowd in the hopes of finding an interesting item for sale.
This has two negative consequences: the crowding of player areas, which causes lag and increases server overhead for the game operators, and it irks players who would rather be playing the game instead of standing around trying to sell stuff.
In its effort to really drive the bazaar system into peoples' hearts, Square Enix introduced Retainers and the Market Wards. Retainers act as a player's personal storage in FFXIV. Just like the player, the retainer also has a bazaar of 10 items, set up in just the same way.
However, retainers can only be seen by other players (and, thus, sell items) when they are placed within Market Wards, special marketplace-esque instances in each major city. There are a number of wards, each featuring a category of item to be sold, however players need not obey these directions and they do nothing to help players trying to sell multiple categorically different items.
Unfortunately, it works rather poorly, especially since the game can only render a very limited number of NPCs at a time. And it doesn't stop players from doing the AFK bazaar thing too. Why limit yourself to an NPC no one will check, when you can also go AFK and bazaar in the streets?
In a nutshell, the Market Ward system is broken. Players don't feel like they're getting a fair price because they have no idea what the prices are: they have to manually check every NPC and player bazaar in order to find out what the prices for items are. Moreover, they can only see prices for items currently being offered by NPCs. If a particular item happens to be sold out by everyone, there's no way to gauge its price.
On the topic of something not being sold, there's no way to know if a particular item is actually being sold or not until you find it on an NPC. If you never find it, it might actually be for sale... you just didn't find it! So, huge time and effort involvement with doing even a small scan of NPCs for available items and price comparison, which all has to be done manually, of course.
And, heaven forbid you actually decide to go back and buy something from a previously seen price list: if you can't remember which NPC it was on or if you can't get that NPC to render... you're out of luck! Incidentally, you can "technically" set up a buy order, but you can only buy what you already have (if you want to buy four healing potions, you have to have four healing potions) and buying takes up more space than selling (both the item you're buying and the money you're offering take up separate spaces that could have been used for two sell orders).
Final Fantasy XIV
What about the provision about external systems? It's happening in FFXIV. Some websites have helpfully designed marketplace systems that, quite honestly, should have been implemented in the game itself. They allow players to list the items on their retainer or in their personal bazaar, list a location for each, and even place purchase orders should anyone feel like filling it.
Sadly, with no API from the game for the sites to draw upon, the information on the website can quickly become dated and inaccurate. It is not at all uncommon to see a listing on the website, only to futilely search to find the retainer in the market ward after that retainer has been removed, or find the retainer without the item in question. Also, external tools like these are only as effective as the players that use them: if only a tiny portion of the player base uses the tools to list their items, they serve a very limited use in finding items for sale.
There's another reason I started with the story about Asheron's Call. AC's economy did quite fine despite the absence of any market system, so why does FFXIV suffer so? It's primarily because of the sheer number of items in FFXIV and the importance of crafting in the game. In AC, there wasn't much to trade; in FFXIV, there's probably several thousand items, and 4 versions (regular, +1, +2, +3) of each.
Because of the complexity of crafting and the inter-craft dependence (it can take 16 components made by five crafters to make even a basic item) players need a rich, robust market system to exchange all these myriad items with one another. This is further reinforced by the limits to inventory space: players can only hold a tiny portion of the available items at one time, thus are forced to sell the majority of what they acquire, either to other players or to vendors.
So what should Square Enix have done? It should have set up a comprehensive market system that allowed players to set both buy and sell orders, just like EVE Online. Moreover, this should be a single, unified market accessible from all three major cities. In FFXIV, players can instantly teleport from one city to another, meaning that setting up three isolated markets accomplishes nothing beneficial at all and merely inconveniences players by forcing them to ask someone in another city to check the prices there and teleport if market conditions are more favorable.
Players should be able to list more than 10 items, and the market system should allow players to simultaneously place orders for equivalent items (for example: you can set an order that buys all items, including the +1, +2, and +3 versions, either for the same price, or for four prices). Finally, this market system should show historical transactions, ideally dating back for a full year (as in EVE Online) but even the last few transactions (as in FFXI) would be acceptable for players to make an informed decision based on both current market brackets and past market performance.
In conclusion, there are a number of valuable lessons all MMORPG designers should take into consideration when developing their virtual economy. First and foremost, give players the tools they need to be an active participant in the game economy without undue burden or stress.
Secondly, ensure that all types of players ("market players", casual players, and everything in between) feel like the market works for them.
Thirdly, make sure to balance all your opportunity costs: don't let the MIMO trickle cause too much inflation. Continual money-out outlets like Player Housing are a fantastic way to keep your economy in check. Finally, remember that you're making a game. It should be fun to play. Empower your players, not just with sword and powerful magic, but with the chance to make dreams of riches on the market a reality too!