I've been buying some items on amazon and ebay recently and looking at the feedback between sellers to determine which to purchase from. Sometimes you'll find stuff like multiple people complaining of an intense smoke smell that was never mentioned in the sales ad, and after 3 or 4 of those you can pretty much expect your item to smell like smoke. What I've seen a lot of recently leaves me feeling bad for the sellers. I've sold quite a few items here on bladeforums, on amazon, craigslist and ebay. I've also bought a ton of items. I have 100% positive feedback on all of the forums I use, so this isn't me lashing out against people who have done me wrong, rather what I see others doing to other sellers.
If you buy a pair of boots and they don't fit - that is not cause for bad feedback. Sizing varies between manufacturers and even between lines (carhartt makes their raingear larger so you can layer under it). It's not the sellers responsibility to determine what size your body is, he/she is selling a known item in a stated size (given by the manufacturer). If you need to know if it will fit you specifically, you need ask for specific measurements before buying. Even worse, when people say they intended to buy a different brand. That is not a cause for negative feedback - you bought that brand knowing it from the sale page, the seller did everything he/she was supposed to do.
I hate seeing this: Neutral feedback: "Great transaction." Don't give neutral feedback without having a reason for it. It isn't about how you feel, it's about legitimate concerns with how your transaction happened, if you got your item in the ebay described ship times and it was the item you purchased - that seller does not deserve neutral or negative feedback. You got what you paid for, on time.
Don't give negative feedback if you just don't like the item. It's not the sellers responsibility to provide you with a product that is higher quality then what the manufacturer makes. If you buy tents that are made in a way that causes them to have condensation problems, that's an issue with that brand of tent, not with the transaction. If you buy boots with a known issue of stitching coming out in just over a month, thats an issue with the boot manufacturer, not the seller.
If your pissed that the knife has a scratch on one side - go back and reread the sale page, look at the photo's before you post negative feedback. If the person says "in new condition. There are some light scratches on the right side." then he told you they were there. If the photo's clearly show that it's got scratches, then you saw them before you purchased it. examine the knife BEFORE you scrub it with a brillo pad or do any kind of cleaning to be sure that if it does have problems, it came that way before you handled it.
Take your time with feedback, especially when you want to give negative feedback. Think of it from both positions, if you were selling something and someone had the same complaint against you, would you still find it reasonable? Feedback is about transaction quality, whether the sale information was accurate, whether you got what was described in the sale ad. If your problems are not related to those things, don't let them change feedback that is supposed to be about the transaction itself.
This is not to say that bad things should be overlooked. If you get a knife that was described as 'new' with no other information and it's edge is gnarled or it's got obvious signs of having been used, then there is often a legitimate issue with the quality of the transaction or the sales description. Just don't tarnish the public reputation of someone if unless they actually deserve it. Do not give emotional feedback at the height of being pissed off for whatever reason, take your time. Be concise. Be clinical about it. A precise description of the reasons someone deserves negative feedback is almost always more informative then someone ranting about how furious they are at the individual rather then the transaction itself. It is also much easier to resolve in a real way, items can be shipped back, refunds can be had, options can be discussed. If you want emotional compensation - your probably not going to get it. Things will probably turn into a heated argument that leaves both parties in a worse place and no one benefited from it at all.