Feedback etiquette should be left in this manner. At least this is the way I see it.

1. SELLERS - Should leave feedback upon payment and arrangements made for shipping. Especially if the buyer pays in a prompt manner and within the guidelines of the seller.
2. BUYERS - Should leave feedback upon arrival of their merchandise. Which should follow the guidelines of proper shipping, packing and the item as described.

I have had sellers in the past who would not leave feedback for me until I left positive feedback for them. To me this is shady dealing. Ebay is based on honesty and trust. If you sell what you describe and the buyer pays for the product you offer for sale then my etiquette should work just fine.