http://ocsnext.ebay.co.uk/ocs/cuip?q...80&segmentId=1
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They can remove it, especially if the seller phone up eBay customer service and they will remove it free of charge.
If you receive an invoice, just video yourself burning it and that will teach them. It's best to write up a blog post or something about the business, tarnish their reputation a little bit.
eBay are always on the buyers side, through anything unless the seller has proper proof. eBay won't remove the feedback (may remove the comment) unless under serious circumstances - threats/abuse etc. and you won't be billed $100 - He is just trying to scare you. You're more likely to get a refund for the item than him get that feedback removed.