1. The players can be able to buy in-game money for real money, this is the only way to print new in-game money. (deposit)
2. The players can be able o convert in-game money back to real money. (withdraw)
3. Rich mechanism to allow players trade all possible itens between them, using the in-game money as currency for the trades.
4. (Optional) The company can use thoses transactions, buy and convert in-game money, to retain a small part of thoses transactions and avoid monthly subscription.
5. (Optional) Players can trade your in-game money by game-time/subscription.
This will prevent black-market.
Without the monthly subscription, players can buy game money to get other players' material, using safe and allowed in-game mechanisms.
This may be appealing to people who wish to make money playing, and again, using safe and permissible mechanisms within the game, without having to turn to the black market.