I have only left bad feedback on one occasion. Bought two estate pipes that were completely bogus crap, stems didn't match bowls at all, were not the Kaywoodies he claimed them to be, etc. These were $5 estate pipes, so I wasn't expecting much, but a complete and total misrepresentation and lie were too much.
Had a guy claim he thought had a Comoys Wimbledon for sale cheap, and it turned out to be a Briar Master Wimbledon. I didn't think it to be a Comoys from the start, but I was intrigued. It is a pipe still regular in my rotation. I took it for an honest mistake by someone who just saw "Wimbledon" stamped on a pipe, and assumed it was a Comoys for that reason. And it didn't go for a Comoys price. Good feedback left.
Recently had a seller fail to include an item (multiple purchase) in the delivery. I contacted them, and with held feedback until I got a feel fro their response. They were apologetic, and had it to me in a few days, nos questions asked. They got the same feedback I would have left had they sent all the items correctly (positive). That was an honest mistake, and they took care of it.
I got bad feedback once, on a sale on my part, when I sold a large piece of home electronics. I sold it at a price, and the shipping was to be figured after the sale, based upon the desires of the buyer. The buyer wanted me to ship it overnight and insure it heavily, which I did, and (my mistake) sent it out on the agreement that he would paypal me the difference. He never did, and then left bad feedback saying I asked too much for shipping. I was ticked that he screwed me on the shipping AND left bad feedback about the cost of it, and he never assumed the cost. Live and learn.