As someone who is now only a buyer on eBay, the feedback aspect of this and making it contingent for a refund would have bothered me.

Having said that, I used to sell vintage posters on eBay and had 4 negatives out of around 480 sales. Three of which came within the first 100 sales I had made. One was my fault, the other 3 were total nonsense but none of them caused any drop in sales. Getting a negative isn't going to hurt you one bit with all the positives you have and I honestly wouldn't give that a moments thought.