I agree, I think economies should be an emergent institution in any virtual world. I understand why professional MMOs don't do it, though: it's a conceptual risk to gameplay. Having to constantly barter for things could drag a game down,
unless the developer is clever and/or spends time working out the mechanics as much as possible.
Which is why I think a MMO would need to hire or consult an economicist. Both to help create basic tools for the economy to grow on (an obvious example: secure trade. Trading would entail more risk and be less used if trading weren't secure (if the other guy just grabs your stuff and runs away.)) How the tools themselves are implemented is up to the developers, but economicists will be able to tell you what real economies need to function properly, both on a microeconomic and macroeconomic level (game companies "mint" materials in their worlds via monster spawn, etc., and hence are acting through monetary policy, which effects inflation and conversion rates with real money wether they realize it or not).
As a more advanced example: crafters (in real life, ie: factories) often use "futures" to guard themselves against price fluctuations, which helps smooth out their bottom line (making profits more predictable) especially if they can then sell through futures.
Basically, party A says they will sell or buy product B at time C at price Q, regardless of the future price of that product.
Wiki.
A little confusing and non-intuitive, but it helps smooth the economy out. Definately not something a game developer would come up with on their own if they're not thinking about it.
I think all kinds of economic activity (from something as basic as simple currency to more advanced ideas) could be emergent if a developer were to construct a basic "contract" object. Contracts could be automatically enforced, written in some sort of scripting language, and be impervious to unauthorized edits, but still allow authorized addendums. If a contractee defaults (say, he promises to deliver product A to person B by time C, but doesn't), it's written into some sort of contract log (similar to credit ratings in real life) so others can check out another person's rating before entering into a contract with them.
Having all the legal obligations automatically enforced as much as possible would help reduce risk and avoid having too many player lawyers (though someone still has to write the contracts).