One thing that bothered me as a CoX player (and also in other games) was that there was this big dividing line between vendors and the auction house. Vendors would have a list of things they would sell, fixed prices, and might buy your trash, while the auction house operated as a completely independent oversimplification of a commodity market or consignment house.

To this day I see no reason for these to be different things. You only need a few things to merge the concepts into a proper store model:

1) Stores are restricted in categories and rarities of items they'll buy and sell. Tech stores sell tech items, magic stores magic items, both only offer rarities up to "rare", whatever.

2) Auction houses filter toward the rarer stuff (i.e. they might not even list commons) but have little or no category limit.

3) For items the devs want to guarantee availability on, stores have a fixed price at which they will sell these items. The store appears like any other seller, and has effectively infinite inventory.

4) For items the devs want to use as "vendor trash", i.e. guarantee a certain rate of conversion to credits, the store shows up as a buyer with a fixed price. Like any player, the store can be both buyer and seller for a given type of item. Obviously not at the exact same price.

5) Implementation-wise, it's really all one big store on the back end. If one tech shop is selling a turnip twaddler for 100 credits, so are all the other tech shops.

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For the record, I'm a big fan of how the old 8-bit game M.U.L.E. handled its commodities markets. For those unfamiliar with that game, each player would show up in a vertical "lane" on the screen. Each would stat at either the top (if a seller) or bottom (if a buyer) of the screen, in an "end zone" the folks on the opposite side couldn't enter (thus you could not be forced to buy or sell).

The players could then move up and down, with their vertical position indicating their offering price. Once two players' prices met (i.e. they were aligned horizontally), commodities would be sold from one to the other until one of them moved away or the seller ran dry.

In addition to this, the store would have a price set at the bottom of the field where it would buy the commodity, and if the store had any in stock, it would also appear at the top of the field with its selling price. This would set a floor and ceiling on the price.

Unless the store was dry, in which case there was no upper bound and buyers could run all the way to the top [i]and keep going[/i] to crank the price way up in the hopes a seller would come down out of the end zone and sell them something.

See it in action in this video starting at the 2:43 mark: http://www.youtube.com/watch?v=ZWqTYKgGx54