I've always found arbitrage to be the best way to earn money in mmo's. I used to buy up all levels of cheap green items in the wow auction house, according to exact limits i came up with and stored in a spreadsheet. then i would disenchant them and sell the byproduct.

It was time consuming, but i made a lot of money and it took a long time before anyone else figured it out and joined in. the problem was it only took a few people on a server to skew the prices of the masses. but by then i had a lordship and manor in every city in the eastern kingdoms.