When I sell something, I leave feedback immediately after I ship the item, so the buyer knows to expect it. If I've received payment and I mailed the item out to the buyer, then I have completed the transaction and therefore is my responsibility to leave feedback.
When I buy something, during the Paypal transaction I leave a note that reads, "Please leave feedback upon shipping and I will leave feedback upon arrival of item to complete transaction."
I've been a member since 2000 and have had about 225 transactions with 183 feedbacks. This was the way I was "raised" on eBay. I think it is a just way and I will continue to do so.
I believe the seller should leave feedback first. The seller should not wait for the buyer to leave feedback, the buyers transaction finishes last and the seller should give the buyer no reason for neutral or negative feedback. If such is left and is injust, eBay will try to work it out with buyer/seller and also that's what the retort feedback is there for.
JMHO!