I mainly work IMVU.com as they have the best profit making system by far. In fact the game was built from the ground up to make money, established by one of the founders of There.com. It costs you about $10 to get setup as a developer and another $20 if you want to have an access pass ( internal adult verfication ). if your serious there is some good cash, but you want to either be handy with Photoshop or something like 3dmax/ sketchup. I just work there as a texture artist and have nearly 50 accounts now, two at Pro level
Think of it as marketing with these 3d worlds and before you do a thing build a home page and ask yourself the same questions you do with domains, how do I get traffic.
All the professional artists tend to just make clothes, so let them have that area

, start with furniture items and just look for meshes that can be changed with only changing one texture. Your best customers are not the men its the women

aged 35 + ie boarded housewives. i sell ingame credits as well and some of these users spend 200-$500 bucks a month in these games, most of the junkie users will blow about $20-$50.
I can't say its easy money because it is not, most of your market is teens, you just got to look for the mums online too

The game has rules and you need to keep to them, so if you want to sell adult itms you can but you need to have a adultpass. there is good money in adult BUT its not the most profitable area in these games.