You can't rely on the feedback 100%. I just had a buyer wanting to return an item even though I do not offer returns, and her feedback is 100% positive - go figure

I'm confused by your question. When you are finished with a transaction you can leave feedback. Underneath the comment bar you can choose positive neutral or negative and also rate shipping, description and the like with a number of stars. So you need to check negative if you want to leave negative in addition to your comments. You can also leave neutral or positive in the same manner. Sometimes a browser will not fully load the page so you might have missed this option.