It's harder than it sounds.

Last time, I participated in such a thing (on a MUD), I had to pay back every copper in one real time week. The problem was that the designers goofed in creating the market, and it became possible to acquire massive amounts of wealth that normally would take a long time to acquire during game play. They decided the only fair way was for everyone to pay it back. In one week. Bleh. That market thing never returned.

Ironically, I wouldn't have acquired the resources to pay back the profit, if I hadn't er, gamed the system in the first place.

A stock market in a MMORPG sounds great, but getting it to work depends on the situation. Most of the time, players' economic needs are real simple (I need a better sword with cooler flashy magical effects). No need to toss in limited liability and the stock market unless there's a lot more sophisticated happening.