If you made an MMO with no "universal currency" (coins, etc), all trading is the barter system... how would players handle it? 
I imagine it'd be heavily crafting-based with all items created by players, no merchants etc
if NPC gives you something it's probably because you did a quest for them
I'd worry about the crafting being too grindy, but it's hard getting around that if you don't want an overabundance of goods
