If we want this game to avoid the economic fate of 99% of all MMO'S then we must agree to keep the amount of money finite and semi rare.
Here are the steps
1.Create a Universal Monetary Value, and Universal Currency. One "Elyer" for example= 0.05 ounces of Gold/33 ounces of silver Again just an example
2.Set up a Universal Banking System The banks would be responsible for simply exchanging G/S for monetary notes, and monetary notes for G/S.
3.Establish a small group of users who over see the system and stop it from imploding. How? By allowing one person per kingdom to be appointed. These people could certify a very small group of people to create/print monetary notes.
4.Printing to demand Its a simple process, the over seers and banks keep tabs on how much G/S they have and how many notes they have. If someone comes to trade in G/S for notes to spend, they check with everyone to see if there is any available, if not then according to a universal ratio of notes to G/S the printers would be asked to print said amount and then the bank would collect and hand over said amount to the player in exchange for the G/S.
5.Once all the Gold/Silver is minded Once its gone its gone, no more notes printed and no other metals used.
THIS IS THE ONLY WAY TO PREVENT INFLATION AND KEEP THE MARKET STABLE
What do you think?
EDIT- Done by players not built into the game