Based on my reading these boards the past few months, you will probably have little recourse if the buyer wants a return.
I've never sold comic books on eBay, but I have purchased quite a few, and I have received a few books that arrived damaged. They did appear to be shipping damage, as there were usually marks on the box, but in one case it was very well packed like you described (gemini mailer with additional cardboard). I do let the seller know immediately and sent pictures of the packing, and they typically offer the return/refund. One did later offer a partial refund in lieu of the return, which I accepted (since I think it would press out). I have never done a 'shakedown' about negative feedback, though, as that seems like bad form. I don't think I would ever initially ask for a partial refund either, as in most cases I'd rather just return the books.
One seller did ask me to send the original packaging back, as I think he was going to file a claim with USPS.