In my opinion it does not make sense for a seller to leave feedback before the transaction is complete to everyone's satisfaction. Just as it would not make sense for a buyer to leave feedback before an item is received.
I laugh when buyers send messages immediately after payment, demanding that I leave feedback right away - as if merely paying for the item should earn them some kind of medal. They are SUPPOSED to pay for the stuff!
I leave feedback after the transaction is complete, and I leave it based upon how the transaction went, and my evaluation of how my trading partner conducted his/herself during the transaction.
My feedback policy is explained in all my item descriptions. As far as I'm concerned it is one of the terms of the transaction. If someone finds the policy unacceptable they should shop elsewhere.
With regard to sellers not being able to leave "negative" feedback, I will add that there are many creative ways of leaving "positive" feedback on a buyer with a narrative that will be constructively negative, and a red flag for other sellers. Some examples: 'Bought item - never paid' or 'Left unfavorable feedback with no prior conversation' or 'Took issue with my terms of the sale' or 'Tried to change the terms of the sale' and so on.
Finally, I think the best way of finding out the "character" of a potential trading partner is to look at their "Feedback left for others". It tells the whole story. When I see a (stamp) buyer who seems to have "issues" with a large percentage of his/her transactions - that member gets an all expenses paid trip onto my blocked list. My favorite jerk was an eBayer who left neutral feedback complaining that the shipping charge exceeded the actual postage cost. Unreal. Of course that situation begs the question, "If you thought the shipping was too high...why did you buy the item?" And then to leave neutral feedback...really. What an a**h**e. Most buyers - in my opinion - are a pleasure to deal with. But there are still a few morons left out there. That's why
ebay invented the blocked list.
Maybe
ebay should run scans looking for user names that appear on a lot of blocked lists. Then they could establish a cutoff number so that if someone is on more than "x" number of blocked lists they are banned for life. The logic would be that all of those "blockers' can't be wrong - this is a troubled individual.