My feedback gets left after I've packed the item then marked as posted when its actually posted leaves me a visible sign tht I've packed the item or posted it.
For selling, I never leave feedback until the buyer does these days, I have been burnt too many times by buyers.
For buying I leave feedback when I am happy with the item, if I am not happy generally I don't leave any feedback, although sometimes I am a bit slack and forget to leave feedback.
