Russia and America are at war with china.
I have good relationships with all three.
I don't want china to be deafeted so I'm trading hores and iron with them.
My question: Will this damage my reputation, or ruin Russia's and America's attityde to me?
It will not ruin your reputation, except if you are dragged in this war before the 20 turns of the deal are over.
I don't think Russia and America will hate you for that. At least, their attitude towards you won't tank. It is not impossible their attitude worsens slowly though.