Classic multiplayer world problem - I want to give you 10 monies for your rare sword. At no point, from any view should I have 10 monies and the sword, nor you

Classic multiplayer world problem - I want to give you 10 monies for your rare sword. At no point, from any view should I have 10 monies and the sword, nor you the sword and 10 monies, and especially bad is if neither of us have either. Also, it shouldn't be possible for someone to lure a goblin over to stab me forcing me to cancel the trade because I died and warped back home. Also it needs to happen within like 200ms because otherwise people will leave for a less laggy game.