This is a complete scam. There's no way for them to transfer the GP any longer because of the balanced trade limit update.
These sites are run by criminals. Do you really want to give them your credit card#?
This is obviously a scam. You must at least provide username. Without username how will they transfer the gp to your account? And, selling runescape gold is against the runescape rules. You will get banned for selling gp.