You do need money sinks, but you can also encourage people to keep the items they receive in circulation, instead of NPCing them. If the benefit of selling an item to an NPC is minimal in comparison to trading with another player, then the economy will be self-sustaining.

Requiring players to pay a fee (whether monetary or farming related) before being allowed to go on a quest/raid with high rewards can also help, along with the other general ideas of charging for services (transport, auctioning, repairing), or consumable items.