some subscription games will sell you things in addition to the monthly fee, there's a general sense that you should earn your advantages. But in free-to-play games that have a cash shop allowing players to purchase items directly... well, the very idea of gold selling seems kind of silly. But it can happen, and Runes of Magic has experienced a bit of a problem of late with the trade of Diamonds (their cash shop currency) for gold.
The game has traditionally treated the currencies as interchangeable insofar as players with lots of Diamonds and not enough gold could sell their excess to other players. Unfortunately, the current spate of RMT activities have called for draconian measures: they've taken away the ability to trade Diamonds for gold. They've also temporarily removed the ability to send gold through the mail. The official announcement stresses that this is a temporary measure to combat unacceptable behavior. While less gold spam is a good thing, fewer features are a negative, so this new is a mixed bag for Runes of Magic players.