You can definitely be a business man and a lot people are. They are running a full gold selling and buying business which also includes other services at times like middleman, questing, and power leveling. The big ones have to take on multiple orders at a time and work with their customers and some even buy advertisements for their business.
So, if you think a person can't be a business man just because RS is a game, you completely wrong.
A lot of people can make services professional because they do it in the most efficient way or they have knowledge on how to do it the most efficient way. For example, a fire cape service may seem easy to you, but I highly doubt you have 100+ orders for fire capes and been successful all the time with different types of accounts. Some people do just put professional on their threads or websites to bring in more customers but there really are professionals at this game.