The feedback is irrelevant in law it is just a means to give someone credibility on Ebay.
You are within your rights to ask for proof of this problem, and once you have that you either need to pay for it to be cleaned or offer a full refund. Look at it from the customers point of view you wouldn't be happy with the problem.
When you are selling things the buyer has a lot of legal power, which is the way it should be, and selling some products at a loss for the seller is all part of normal business when their is a problem.
When I sold on Ebay for a couple of years I'd correct any problems whether they were caused by me, the post or the seller at my own expense as this results in more sales in the future.
