I've never left negative feedback on Ebay. I live in a world where your good name is more valuable than any money anyone would offer for it, and I consider trashing a person's reputation to be the nuclear option and a final resort. I won't use it unless a situation is just untenable.
I have, however, used the Ebay dispute resolution process several times when I did not receive what I bought or when the item was substantially different from the way it was portrayed in the sale. I have always gotten all of my money back, including shipping. In one case I got my money back and then the seller contacted me (finally, for the first time) and offered me the items for much less than I originally bid. I accepted his offer and ended up getting two side covers and a sprocket for the cost of shipping. His reputation was important to him.
Ripoffs are ripoffs, but I think some sellers on Ebay are not malicious, just lazy, and they will make good given a chance.