Your RS account and any items/cash on it are property of Jagex.
No law applies to the selling of RS currency/items - from a legal standpoint it was neither theirs to sell, or yours to buy.
You still compromise your account by giving them your password and bank pin, no matter how trustworthy they are. Their business revolves around getting their hands on gold to sell, and in the past these type of businesses have gone to measures like running sweat-shops, stealing gold, and accounts. Would you trust them with your account? Not if it's worth anything.
They are unregulated, and do not own what they are selling, and neither do you once you buy it, so you aren't buying gold/items - you are buying their promise you'll get the gold quick and on-time. You have no way (without cooperation from Jagex) to prove they broke that promise.