I'm still playing a lot of World of Warcraft, though I've gone off on a recent tangent from straight playing to auctioning. By playing mind games with supply and demand, I've carved out a few monopolies for myself in the online auction house and my character's net worth has gone from 8 gold up to about 65 gold. With a reasonable time investment and math skills, it's quite easy to make tons of in-game cash -- just as in real life, most people don't understand or care that buying 5 of something for 1 gold is NOT as good a bargain as buying 20 of something at 3 gold. I also put up with people who try to greatly undercut my prices, not realizing that I'm just buying their goods and reselling them for higher prices.

It's like eBay without any lasting penalties for royally screwing up.