In short People would be able to put RP in trading box, and if your account (with whom you are trading atm has enough rp) upon successful trade RP automatically goes to other trader with -20% (tax). Easy and secure. This box would use the same technique as in in-game EM mall, where u can buy scrolls and other stuff as well as check your RP balance.
Requirements: 2 people Party, consisting of both traders (in order for multiple trades at the same time with the one you need out of all other traders).
Of course design may differ, but the solution to scams would be awesome.