Unfortunately, sellers can't expect buyers (or eBay) to be sympathetic to reasons given for cancelling sales. This is eBay, and unfortunately there is not much room for trust. All too often items claimed to have been damaged later reappear for sale at a higher price.
I'm afraid it's almost inevitable that buyers will leave negative feedback, as they're entitled to.
eBay is unlikely to help. The only reasons they accept as valid are that the buyer requested to cancel or problems with the buyer's address. Otherwise the seller is liable to be penalised with an account defect.
As has been said, about all that the seller can do is to apologise to the buyer and send them a photo of the damage. If you haven't done so already, you could still do so by sending them a feedback revision request.