One of the problems with most feedback systems is the fact that there are only 3 options. Good, Neutral, Negative.
I prefer a system where we can rate different aspects of the trade out of ten, or like eBay now do for sellers with stars out of 5.
I find that that would give a much better real world indication of what someone maybe like to trade with. Unfortunately it could be lengthy and complicated.
I also find it disturbing slightly that the expectation is always for Positive feedback. I have, on occasions, left neutral feedback, not because anything was particularly bad, but nothing was overly good either. On one occasion, on leaving a neutral feedback on eBay, I received a tirade of abuse which went on for days. So naturally I updated the feedback to include the developments, which sent this chap into a frothing pit of fury. To me it seemed utterly futile and pointless and only served to paint himself in a slightly worse light than necessary.
I tend to ignore neutral feedbacks I see, they certainly wouldn't affect my decision to buy or sell. Infact, I think a Neutral should be the default setting, and you should only go either side if a trade has gone particularly well, or badly.
As to sellers leaving feedback. Personally, as a seller I leave feedback first, once I know that the item has been received and that the trade has been completed. Then I will have an idea on the buyers communications and genuine intentions. I don't feel that a buyer has completed their part of the deal by paying. They haven't received the goods yet, so the trade isn't over.