The ebay feedback system is constantly being change. I managed not to get a single negative feedback for the first few thousand items and then alas one day it appeared, rather than contact me and ask more advice the customer gave me a negative.
It is very hard to avoid negative feedback, and the ebay feedback system is changing to make sure that if you are giving a negative feedback that really do deserve it.
However if someone had a large number of negative feedbacks it is best to avoid them.
If you do get a negative feedback and cannot get it removed, all is not lost as you can respond to the feedback so others can see your side of the story.
However, please make sure to wait until you are relaxed and calm. Do not show anger in your response, simply explain your point of view, it might even increase sales.
Sometimes negative feedback might occur as emails to a customer are lost in a junk/spam filter. It might be best when emailing certain internet providers to contact them through the ebay contact member box so that ebay will email them directly.