RWT or Real World trading or real world item trading is used to refer to the act of any kind of trading outside of Runescape in exchange for Runescape services or items, or vice versa. If you look at it legally, all the items in Runescape are the property of Jagex which is why it cannot be seen as the personal property of any player. So if a player involves themselves in RWT, they’ll be violating the rule 12 of the game and risk getting banned from the game permanently without any warning.
Why Jagex thinks RWT is against the rules?
Most of the players that involve in RWT get money and other in-game items using devious means like bots for gathering money and other items. They also exploit in-game bugs and try to scam other players causing them a great deal of loss in terms of Runescape gold and OSRS gold.
Since RWT is against the rules of Runescape, a lot of players try to find legal loopholes like claiming the real money has been paid according to the time spent, that there is no relationship between transfers of virtual and real items and more. However, Jagex still considers all these methods as being against the rules.