There are few improvements to be made on EVE Online’s economic tools. The more your system resembles that, the better.
Buy orders are a key feature. Most systems lack this, but it is the “Buy It Now” equivalent for the seller. It would improve economic efficiency enormously. Creating a market is hard, and letting either buyers or sellers make the first offer will encourage more use.
Auctions can sit alongside consignment sales and buy orders, although the buy/sell will set ceilings and floors for any likely activity (reference eBay). It is probably easier to pick one or the other; I obviously favor buy/sell instead of auction. If you are going “auction only,” at least apply the buy order principle and allow reverse auctions. Bob wants an Exceptional Cotton Cloak, and he has posted 50gp towards this. Which crafter is willing to make it the cheapest? (There are some issues here, notably that every bidder should have said item when bidding on the sale, which might lead to excess creation of these Cloaks, or few bids on items where the original creation is expensive. Who wants to risk getting stuck with an Exceptional Cotton Cloak, just for the chance at making a sale?)
History is valuable. City of Heroes hits the minimum for this, showing the last five sale prices. EVE is obviously the best, with great financial tools. Many dislike approaching the market in a state of ignorance. The only prices we can see are for items that have not sold. Any grounding will be useful, even if that tool can be distorted.
Apply this to items not presently on the market. I should be able to place a buy order for something no one is selling (or everyone has sold out of), and I should be able to see history even if no one is presently buying or selling. Again, starting markets is hard, and many will feel sure they just got ripped off.
Most auction houses abstract away geography. EVE does not: the need to ship between stations is built into the game. You can make your living as a trans-galactic trucker. For most games, having all auction houses linked is probably a good thing.
Some people may want to run their own shops, or higher NPC shopkeepers to do it for them, but most of us are happy to have a one-stop shop when one interface reaches everyone’s buy and sell orders.
For other notes on encouraging buying and selling between players, I recommend the official RMT market for Yohoho! Puzzle Pirates (and presumably Bang! Howdy). That works strictly on buy and sell orders, and you can see the margin between them. (Note for your own economics: the more active a commodity, the closer the buy and sell asking prices. Rarely sold items will show vast disparities.)
What are your thoughts? What do you like and dislike on the market?