@julianjaego wrote:
How could a buyer who has not paid and actually cancelled his order be allowed to give a feedback,
Getting past all the comments above and what you may or may not have done wrong in replying - the buyer should not have been able to leave this feedback and you should apply to ebay to have it removed, even if it is only for the offensive language.
If you are persistent and point out that it is against ebay policy, they will remove it. Unfortunately you have to use "live chat" these days, but it is worth the effort, in my opinion.
