I'd never even heard of the game Entropia Universe until a few days ago. After hearing that a player named Neverdie sold a piece of land in game for 650,000 dollars, I quickly started looking into it. Apparently it's an MMO that features a real economy. You exchange real money for Project Entropia Dollars, or PEDs, which can be cashed back into real dollars at anytime.
So how did this player manage to earn over half a million from playing? Back in 2005 he purchased an asteroid for 100,000 USD and over the years slowly built on it and turned it into what he calls "Club Neverdie". Other players pay him PEDs to be able to hunt or mine on his property and other rent small areas for themselves. He reportedly earned back his 100,000 after 8 months. He then sold Club Neverdie for the 650,000 dollars.
The game itself is set in a scifi futuristic universe where players colonize areas and explore planets. It functions much like normal MMOs where players can trade, craft, or fight creatures. It was started in 2003 by a Swedish company named Mindark.
If you're wondering why a game where players trade and earn such large amounts money is relatively unknown, well I don't know. You'd think the word would have been spread back when he spent 100,000 on the asteroid instead of now.
If you're thinking about giving it a try, there's a catch. While you're able to play the game for free, you start off with very little. The things you're able to collect are very cheap and aren't going to rich you rich, in game or out. If you want to kill tougher creatures or collect expensive items you have to buy PEDs to help you start out, and even then it takes a while before you're going to be able to "make money" off the game.
If you want to give it a try, here is the link. Good luck.