GTE CyberTrust Global Root
MacBook Pro, OS X Mountain Lion (10.8.2)
This is not related to malware of any kind. The problem is a certificate mismatch. and it's probably a temporary problem with either eBay or Akamai (who is probably providing some of eBay's services). You probably just need to wait for the problem to resolve. (You should be perfectly safe to click the Continue button in the meantime.)
Similar problems have occurred for other sites in the past. See:
http://reviews.cnet.com/8301-13727_7-57538839-263/safari-users-hit-by-facebook-c ertificate-error/
Thanks. New to the Mac world. Takes a bit of getting used to after 25 years on a PC!
GTE CyberTrust Global Root