"Response: After Archmaester Marwyn is killed, resolve the "when revealed" effect of any plat card in any player's used pile"

So if I were to pick my opponents "Cit of Whatever...." plot do I resolve the effect by using the amount of City Plot Cards in my used pile or his used pile?

should be your used pile.

Always resolve effects from your own point of view, unless otherwise specified. So "you" and "yours" refers you, the person resolving the effect, not the owner of the card effect you are resolving.