There seems to be a lot of thought about who should leave feedback first. I'm not a professional seller or buyer but have traded about 175 items over the past few years.
I think feedback should be left when both parties are satisfied and to my mind this can only happen once the buyer has safely received the goods and is happy with them (after paying for them of course). I think part of the ebay experience is how we all deal with each other throughout the process and if something was lost in the post or had to be returned, I would want to know whether the ebayers had dealt with it all reasonably.
When selling, I leave feedback once the buyer has confirmed the goods have been safely received and that they are happy with the item - this could be by email or by feedback, it doesn't matter to me. There's nothing dodgy about that, just common sense. I wrote a guide a few years ago for people worried about sending out items that were reported as not received, but they had in fact received them and were just getting a refund. My idea was to report this in the feedback, although it hasn't ever happened to me, I would just leave the feedback as normal (obviously positive if the transaction was pleasant and smooth) and would add something like, Item lost post but refunded/replacement sent, all ok. If everyone did that then if a buyer did it a few times it would show up, or would stop them.
As a buyer I wouldn't expect the seller to do any different, although I know a lot of people do leave feedback as soon as they have the money.
If I see something I want to buy, it doesn't put me off just because there are a few negative comments about them, it is the content of the reply to the comments that sometimes put me off. When you read the feedback you can see at a glance if someone is just wanting to get into a slanging match and I wouldn't buy from them at all. But if a seller has been left that kind of feedback and has replied very reasonably then I would probably buy from them.