Back when eBay was becoming really popular, there were reports of people with "eBay addictions," and I couldn't really understand the phenomenon. My dad was pretty hardcore into eBay, but mainly just to collect things and make a side profit, mostly Magic: The Gathering cards. Now I'm beginning to figure it out.
I now spend about 20 minutes a day in the auction houses in World of Warcraft. I find it quite amusing. It's my own little side business, except with fake money, fake money that I can actually use to buy in-game things. I'm part of this bizarrely elaborate economy, where the value of my alchemical transmutation really does vary with supply and demand, and where the savvy buyer can pick up enchanting reagents at cut-rate prices if he knows where to find them. I'm now generating a steady trickle of income, just like the spam says I can, and I can use that income to buy things for myself and friends.
Back in my freshman year in college, I was playing PimpWar, an online web-based real-time economic simulation of sorts (a little descriptive whitewash never hurt anyone). I guess this is basically the same thing. There's the thrill of profit and loss, without any potential for significant impact on my life. The only difference is that PimpWar was free, but not nearly as pretty.