In fact, it is possible to sell in-game services for real money. This means selling virtual services or items inside a game against receiving real money from the buyer.
The method is a part different from many others, but this one should not be fooled. There are many who are actually happy to pay others to make them better in a game and this is what you can earn money online.
You can choose to contact any buyers inside the game or try to find different forums where people can contact each other regarding both sales and purchases. The potential for earning money is actually fine, but you should not expect to be a millionaire on it.
Examples of games that many people earn money on could be World of Warcraft, League of Legends and Dota-2. If you would like to earn money online with this method, you should clearly try yourself in one of the three aforementioned games.