As a seller, I leave feedback as part of the shipping process. As a buyer, I leave feedback when the item arrives. To me, that's the way it should work. I have never had a zero feedback bidder win one of my auctions, so I can't comment on how often that becomes a problem.
For a return, the feedback option should be disabled so long as a refund is given within x time. Anything other than this can and will be used as manipulation. As far as even getting returns, there will be hardly any. People do not want to go through the hassle to send it back. Ask the marketers who came up with the mail-in rebates.
I used to be afraid to leave sellers a neg as I didn't want one back as a buyer, I have in my 16 years using ebay left 2 negs for just that reason it would likely have been a dozen otherwise.