It is possible to. The gold is acquired by "less-than reliable" means botting, scamming, and account theft. It is then "delivered" into the buyers account through a different account. And of course, there is the chance of being taken advantage of or even being reported for fraud because this is "real-world trade" and dropping or picking up or the transfer of a huge quantity of gold at one go could be... like a scam.
BTW, "real-world trading" is against the rules of Runescape and Jagex will beg you to clap. The only legal way to "RS Gold" is to pay real money for the Bond (an item that Jagex sells) and selling it to a different player in exchange for gold that is available in the game.