I had a fairly new buyer ask me a few months ago how I would package a particular item because she was worried it would get broken en route, as she had had issues with that previously and the seller refused to respond to any of her messages (she wasn't aware of opening cases and that kind of thing at that stage). She even offered to pay extra for postage to ensure I had enough supplies to package it properly. I assured her that I had plenty of packaging material here (I keep good boxes and packaging from things I buy) and no extra payment would be required. I've had enough things broken en route, like 4 plates I bought that were each wrapped in a single sheet of newspaper, then put into a 500g satchel bag with no other packaging - completely smashed on delivery - that I wouldn't knowingly do that to a buyer buying something that could potentially break. I told her in the event that it did arrive broken, take a photo and send it to me and I would give her a full refund of both the item and the postage. After it arrived, I got the loveliest message from her thanking me so much for having common sense and that common sense seems to be lacking these days. She even felt I went overboard with the packaging and I said no, that's how it should be with things that might break. For days, I kept getting messages thanking me. 

Unfortunately, in any business, there are people who think they can scam people. In my opinion, no proof, no refund. There are too many keyboard warriors who are big and tough behind their computer screen because it gives them some sense of power and sadly, we will all encounter those people from time to time. My guess is, whatever you sent WASN'T faulty or broken, they were just trying to see how far they could push you and whether they could get their money back, even though it was only a small amount. Most buyers who did receive something broken or faulty would at least have the decency to send a photo or some other kind of proof that there was an issue.

One last thing, threatening to leave negative feedback to essentially blackmail you is against ebay's terms and conditions so I would forward that particular email to ebay. I'm not sure if there is any way to know what these people are up to prior to sale, apart from checking their feedback they have left for other buyers. I cancelled a bid a month or so ago because out of 20 feedbacks left for other sellers, 15 were red and 4 were grey. Only one green. I wasn't risking my reputation for someone I felt could've been a scammer.