about ten years ago, i sold a course textbook on amazon and the buyer claimed to never receive it. i refunded his money, but he still went ahead and gave me a one-star rating... now, in the years since, pretty much no one bothers to leave feedback for me, at all, ever, on the media they buy from me... so i *still* have a not-fantastic average seller rating. i even emailed the guy a couple years back asking him to please rescind the rating, as he did get all of his money back (and i was out my book and shipping costs) and he did agree to do so, but then wrote back and said he was unable to change the rating. hooray.
(i have an otherwise perfect rating, and i *do* have a perfect rating on ebay/half.com, so i am decidedly not a scammer...)
That was a very sucky thing for the buyer to do, giving you a one star rating even though you refunded the money. The fault could have easily been with the postal service. I've had books I've ordered through the Amazon Marketplace not show up, but each time I've contacted the seller they have refunded the money and I give them a five star rating with an explanation in the text of what happened. I know that I might be giving too much credit to the seller in some cases, but I figure that if it really was the fault of the post office (like it was in your case), the seller shouldn't be branded with a bad rating. In fact, because they did respond promptly and gave a refund, I have more confidence in them. After all, at a minimum they are out the refund cash, and most likely also a book and postage.