Jagex, creators of RuneScape, have said countless times that it is PROHIBITED to buy Runescape gold with real world currency.
The act of exchanging virtual goods or money for services/real money or vice versa is known as real world trading. This is strictly prohibited, as clearly stated by Jagex on their website:
"Game items must only be exchanged for other items/services within the game." (Real world trading)
Real world trading is detrimental for Jagex and it does not benefit the company in any way. It is also illegal in a quite a few senses.
All RuneScape accounts and in-game items belong to Jagex therefore selling these things for real currency is illegal - you cannot sell what doesn't truly belong to you:
Nobody has our permission to sell RuneScape accounts or any RuneScape related virtual in-game item (Why do we have this rule?)
By engaging in real world trading, the RuneScape economy also becomes devalued. As items are easier to obtain, their value goes down which makes it harder for legitimate players to make any sort of profit.
Engaging in real world trading breaches the Terms and Conditions that you will have agreed to. This means that Jagex will be able to completely ban your account and do whatever they please with your account:
Any attempt to real world trade is a serious breach of a player’s agreement with Jagex. (Why do we have this rule?)