If it was me i wouldn't be interested in anything but returning the card for a full refund. The card may still be worth 75% of its original price, but i would not have been interested in it or bought it in the condition it is. I think most people are not interested in buying damaged cards even at a discounted price.
As for leaving negative feedback, i think the seller is responsible for any deals they make whether it is for them or their friend. They have ultimately made that decision and if there are problems with the deal then it is their problem and a chance they have taken selling for a friend. If there ends up being a problem with the deal then they should work it out or wear the negative feedback. As someone who has had all positive trading experiences and feedback on here but less than 20 feedback score i assume if i was in the same situation and said i had been selling for a friend that i wouldn't be given the same leniency.