Extortion? will it stay or will it go? Update: comment removed, neg stays

Some board posters see this as extortion.

I recently sold my daughter's old Nintendo DS. I looked it over pretty thoroughly before selling it (listed it in "fair" condition) and included an option for a refund if my listing was inaccurate. Today I notice the buyer gave me negative feedback for a hinge that was broken. Huh? I don't remember seeing that! I'm usually really careful to list anything wrong with an item, but regardless gave the buyer the option for a refund. I contacted the buyer and the buyer writes back that there is a crack in the hinge and if I refund $20 she will remove the negative feedback. What should I do? Do I have any recourse?

Here's the rule:
The buyer isn't allowed to threaten to use Feedback or detailed seller ratings in an attempt to force a seller into providing:
Goods or services that weren't included in the original item's description or purchase price
A refund
A return

Here's the neg:
Right hinge part was broken; Did not mention about it.

Here's the link: 3 Mar 11

Updated:
Comment removed, but the neg stays.
That's just odd.

Seller adds:
Thought I would post a follow-up in case anyone is interested. I called eBay this afternoon and they agreed that this was indeed a case of extortion and have agreed to remove the feedback.

Also they say they will look into this buyer, so hopefully they can shut him down, if only for a little while.

Thanks for all the suggestions. Hopefully by doing this I've saved at least a few people the trouble I went through!