feedback for buyers

[friesian]

How come  feedback for buyers can only be left as positive?, I just had a very unfriendly buyer who did not read the description of the item  then was not happy and when I even offered to accept a return minus shipping cost they promptly left me negative feedback, ( my first ever in 20 years of ebay selling)  called me dishonest and said that I never offered a return.... I asked her to please go back and read my first message to them where I had offered a return and got a rude reply, So, I even I re- offered a return should they revise the negative feedback they accused me of harassment and said they would " report" me..  I did make every effort to be kind  I should be able to warn others, but I can only leave this person positive feedback. I did report them to eBay as asked that they review and take down the negative feedback but nothing has been done, all I can do is block this buyer from buying from me again.   I have made every effort to work it out with this buyer but am frustrated that she was able to leave me negative feedback before even making any effort to work it out with me and I cannot leave anything but positive..