I'll let you know in a couple months as this is what I'm currently doing. However my game is a casual game. Not your normal MMO where you have a ton of NPC's etc. My game is more like real life where you chop trees for wood, gather berries, farm corn, wheat, etc. There are zero "NPC's", I will have animals like cows, deer, etc that provide raw matierals like meat, fur, etc.
The players buy land plots and can build things like a bakery, lumbermill, even a marketplace where other players can put up their goods for sale and the owner takes a % of the sale. All this is 100% player driven, I just randomly seed the map with these resources and the users are able to do things to replicate the resources (breed, plant, etc) to keep things moving. The players have certain needs just like we do which will force them to collaborate together to meet. One person can't possibly do everything to meet their needs.
Alts? Sure but then they'll have to meet their alts needs too. It just won't be efficient to do that. The needs are things like a VARIETY of food, drinks, clothing, & entertainment. Each need has sub-needs that make up the greater need which makes up the players greater "health".
This is sort of like an experiment to see how players play the game. Could it completely backfire on me? Sure, but then again I'm doing this for fun.
A think a stable supply and demand system will eventually come out. At first prices might be high but as more resources are gathered they'll come down. At first the % to sell something in someones market will be high, but someone else will make another market and undercut the other market and so competition will start to drive prices down. As with the real world the ones who get there first will generally get the rewards.