Good points there Pedro, I have noticed that some buyers won't leave feedback until after you do, so if you buy the item and pay immediately you should have a positive feedback, but they wait, so my guess is that if they stuff up and you leave them negative feedback, they will leave negative for you.
Unfortunately, we are considering leaving negative feedback for a seller here in Australia, it appears to be not what was advertised, and they won't respond to our communications, will try once more to communicate and if they don't respond, then, so be it.