We did once consider selling runescape money and items in an online shop on our own site for real cash. However we decided to sacrifice that extra revenue for the good of the game, since we felt it would devalue hard-working players achievements if another player could then just buy their way past them. We don't want rich players to be able to simply buy their way to the top. This is clearly not how the game is meant to be played.
There are also practical reasons why we can't allow this. For instance if we allowed players to sell accounts outside of the game it would become very difficult for customer support to tell who the rightful owner of the account was, which would undermine our password support service.
We are checking auction sites such as Ebay on a daily basis, and all auctions found there are removed, so if you try it you will lose your money. We are also making all efforts to track down buyers and sellers of items/accounts and will ban any accounts we find linked with these auctions.