I have not sold anything in a very long time, but I don't think you even have the option to leave a negative feedback for a buyer anymore, period. I could be wrong though. It certainly should be allowed w/o fear of retaliation if you win a non-paying case, but I don't believe ebay has advanced that far yet.
I still have not seen any improvement in the feedback field either. It seems to take forever to leave feedback as a buyer and I can only leave 25 at a time (even though there is an option to expand the list to more - i just get error messages). I select the feedback, leave the message and then wait for the page to update, which can take several minutes per page. it is not my computer, it is the site. I also frequently report listing violations as they relate to me seeing unnecessary crap in my searches (key work spamming) and nothing ever happens. I have reported several sellers who have hidden text. In one example, a guy is selling a modern collectible lunchbox with a few current Dodgers on it and if you click/drag anywhere in the large blank white area under the description, you'll find a list of 1000s of Dodger names typed in white that you can't otherwise see! The guy is hiding it to get keyword search hits and they still don't care, even though that is specifically against their own rules!!! Ebay doesn't seem to care about operational issues, but they will roll out these unwanted and unnecessary changes to everything else all the time just to keep things changing! Ebay is still the only real game in town and I will use them as needed, but they have room to be much better if they wanted to do so.