1) Have the rwt'er have a semi long skillcape and their money
2) Take both people near an exit
3) As your about to deal the final blow the the trader, stop attacking, let them activate their skillcape, and leave
4) You will be outside and they would have JUST DIED
5) Wait 30 seconds, and reenter, run to the spot you have killed the seller at, and you have no penalty, leave and you've successfully transffered items.
This is better than leaving as your opponent dies, as it gives you an extra 7 or so seconds to get to the items.
Enjoy
`Jared