These days you cannot log in to any MMO without being spammed in one way or another with offers to sell you gold for real world cash. It is now considered commonplace in massively multiplayer games, but it wasn't always that way.
This post was written a few years back when the gold selling industry really started to take off. It's a pretty interesting read, explaining how these people and companies made inroads into our games, and how even now most of the sites we visit are generating money for the same people who are stealing our accounts, and selling the gold back to us.
The question of whether or not gold selling should be against the game's rules is a tough one that ultimately is up to the developers to decide. Unfortunately it's becoming more and more obvious that the developers aren't against it in principle, only against it because they want exclusive rights.
After all, even if money did grow on trees, creating it by typing numbers into a database is probably more profitable.