I recommend you seriously consider carefully the feedback you intend to leave as a reply to feedback received.  Type the comment into a word.doc and leave it for a day or two.  Go back and read it many times and see if it needs tweaking.  Once it is left it is almost impossible to have removed.  If you are looking to eBay to remove the feedback that was left for you then you should let that process run its course before replying to feedback.  This should include all avenues (emailling eBay, phoning eBay).

To leave a reply to a feedback received you go to the "Community" menu heading at the top right of an eBay page.  Hover over that heading and select "Feedback Forum" at the bottom of the list.  Look to the right of the page that loads and choose the "Reply to Feedback Received" link and choose the feedback to reply to.