eBay doesn't police it (much like YouTube doesn't police their own site for infringement) except to the extent it takes to ensure the sales follow their policies regarding eBooks here
If any of the sales are illegal it's up to the IP rights owner to issue a takedown notice to eBay. eBay has the same policy regarding counterfeits. If they haven't done that then the listing stays up, as eBay has no way to know who has rights and who doesn't.
Source: I used to work support for eBay.