Economy Design

How will I go with making my Economy for my MMO game, by adding money of course. My currency will be coins and jewels for the players to shop and trade for items. If they needed some right now instead of earning them from battles, The players will use real money to buy some currency to have and shop at the store. The stores will be running by NPC instead of players because they can be merchants sell, they products at HUB levels. When Gold farmers sold up, I will revoke their logins and block them from playing the game.