Buyers cannot use feedback to extort sellers. If the buyer was clearly saying, "refund me or i will leave a negative", then you have a case of feedback extortion and can probably get that neg removed. But you will need to get a hold of customer service, perhaps several tries. The customer service call center was closed down due to the pandemic, and re-routed to email, chat, Facebook and Twitter. However, some calls are now getting through, or you can choose to have CS call you in some cases. Use the Help & Contact link on most eBay pages to connect with a rep, or go to eBay's social media platforms.
Ok that said, you may be able to use a technicality to get the neg removed, because otherwise, it looks like this buyer's dissatisfaction is on you this time. Your condition report in your listing is woefully inadequate in describing the flaws in the item. Book collectors are very picky about the condition of the dust jackets and book in general. There are not enough close-ups disclosing the damage. And it's never a good idea to send an item out with dead bugs.
Your response to the negative sounds defensive. The seller's follow-up response is one of the most valuable pieces of real estate on eBay, with the power to completely disarm the negative effect of the rating. But it must address future buyers. Your comment will mainly connect to other sellers who understand what you mean. To a prospective buyer, it just says you want to invalidate the buyer's concern. What it needs is to show that you acted on that concern. If it said something like "Buyer declined a full refund" then at least your audience knows you were responsive to the customer. Responses need to be offensive, not defensive.
I know it may not feel like it, but you got lucky here. The buyer could have opened a Not As Described claim and you would have had to pay return shipping and a full refund. I agree with some of the other posters. I would have refunded without a return in this case.
