yes, this is done in many of the popular MMORPG games like Runescape, WoW, etc. You can even set up a website for this where players can order and just spam your link in the chat saying something like if you need gold buy it here. You can also help people level up their character fast for money. If you have a high level character, you can add them to a party and share experience with them. Some people pay big bucks for you to level them up. They could pay like $100 for 20 levels.