I've been trying to design the economy for a specific theoretical not-so-massively multiplayer game - something to the tune of <5k players. It's actually quite difficult. If you don't want to read my thoughts, please skip to the last paragraph.
Difficult, yes. Not the faucets and drains - I won't claim those are easy, but they are, at least, straightforward and easy to put preliminaries in place. Adjustment during testing is critical for those, but they are well understood by me.
No, my difficulty is with the marketplace.
Games have a lot of different kinds of ways to handle this. It's not just what role NPCs play, or whether a player has to be on-line to hawk his goods. It goes deeper: what is the actual structure of the market?
Most games use geographical market location(s), such as auction houses or merchant districts. Some use a kind of world-spanning market communications array, allowing people from anywhere to market anything.
The first option is good in that locations are great. If you're selling something in a location, the thing is right there, the thing can be inspected, and, moreover, it causes interplayer interactions - and being social is good.
The second option, however, offers an ease and 'on-line shopping' type of slickness. Even the newest of newbies can sign on, find something to buy, and buy it. Sure, it might have to be shipped from location to location, but basically, it's point and buy. This also allows all people equal access to the market. You put it up, it gets put up with everyone else's stuff. Also, it is really easy to continue to sell stuff while you're off line. The downsides are, of course, the complexity of shipping and the lack of a social area. In addition, if the market takes off, you can end up swamping it and requiring all sorts of refinements and filters to get anything useful out of it.
My theoretical game is small user base, which suggests that a (mostly) universal market might be the way to go. But I'm looking for more info. I'm very happy for advice and/or links on the matter. Remember: being obsessed with emergent games, this theoretical game contains highly diverse (often unique) player content.