I appreciate that events in-game brought you to the forums today but using the forums to complain about the conduct of another player is not welcome. For this reason, your thread is now closed and the opening post removed.
The best (and only) way, to deal with a situation like this, is to report the player, soon after the event, in-game. Your report will capture a brief snapshot of all recent interaction, including completed trade screens and chat window dialogue. This is the only evidence that Jagex accepts.
Sadly, because there is nothing more that can be done via the forums, please avoid posting this topic again. For more information to avoid such a pitfall again, I recommend that you read the Scamming prevention guide page of the Support Centre.