The Ebay seller could not leave negative feedback for a buyer anyway. They can only withhold feedback. So the only real damage would be done to the seller, not to the buyer. I think that is sad, that sellers have no recourse anymore. big sellers get lots of bad feedback, but because they are big sellers with lots of items the bad gets lost quickly and does not seem to affect them too much because of the volume they sell. It is not Ebay protecting them.
I am wondering, I seem to have missed whether he got insurance or not. If he did, you may need to take the package down to your post office and have the damage documented. They will determine if it was their fault or his for the way he packed the machine.