If they're reselling content for which you own the copyright, you can file a DMCA notice to report their auction listings to eBay for copyright infringement:
http://pages.ebay.com/help/community/registered-agent.html
Unfortunately, we have no way of knowing who a buyer is until after they have already completed their payment, especially with eBay sales, where orders aren't even initiated from our system; we only find out who the buyer is after the fact, when PayPal sends us confirmation of completed payment. Even blocking a given IP won't work for long, as it's trivial for the buyer to get a new IP by moving to a different wifi hotspot or simply resetting their router, and the blocked IP would eventually get reassigned to other users of the same ISP who may be legitimate buyers.