Runescape sent out the warning to prohibit in-game currency trade.

20 December 2006 - A Christmas Reminder   
As the festive season quickly approaches, we would like to take this opportunity to remind everyone that purchasing in-game items or RuneScape gold for real money is completely against the rules and strongly discouraged.
For the safety of players¡¯ accounts, it is very important that well-meaning parents who may think that this would be a nice Christmas present, are aware that any accounts involved run a very high risk of being banned. As such, please do not accept or give away any gift of this kind.

To help spread that Christmas joy, try to find a more suitable present; something that says ¡®Merry Christmas¡¯ in a way that no amount of gold pieces or rune armour could ever compare!

If you would like more information on Rule 12: Real World Trading, please see the RuneScape Rules of Conduct.

We would also like to reassure players that we have a very safe way of detecting this (by tracking back to known real-world traders). We DONT just look for unbalanced trades! So there is no need to worry about being banned unfairly. We are extremely cautious to avoid that, and are aware people often lend each other items, etc...