Scams are best reported in-game at the time of the scam taking place. This records chat and trade evidence for Jagex's player support staff to review and take appropriate action on.
To report ingame, though, the report must be made at the time the scam takes place. Given its been a few hours since your forum post here you're outside of that window. You can, however, send information that you have to the email address '[email protected]'. This is a no-reply email address so you won't hear anything back, but it will allow you to submit the information for review.