I certainly will not go against the wishes of the admins of this site, but I do have a bit of a different opinion regarding this issue.
Here is an example:
A person is selling an item. Another member contacts him by PM and asks several questions. Eventually a price is agreed upon and the buyer says he will purchase the item for the agreed upon price. Now the buyer flakes.
Illusion's position is that there never was a transaction, so there can be no feedback on a transaction that does not exist. I see his point that hundreds or even thousands of times this happens with online buyers. My point is that a deal has taken place. The buyer committed to buy, but did not follow through with his responsibilities. To me,this warrants negative feedback.
An example where negative feedback is not appropriate is for lowballers. If I am selling an item for $100 and someone offers me $15, then I can ignore it or even tell the offerer "no". This is not a reason to apply negative feedback.