I get that on my mobile sometimes. I don`t know why it comes up (someone will tell you) but I don`t think it means anything serious. I just go ahead anyway. I have had it on some applications on our work internet (huge, established company) and I would think it`s not a problem on ebay.
My mate is in IT Security at work and I would ask him but he`s basking in St Lucia but just looked on my (unofficial) work forum and someone said it`s just asking you if you trust the site.
I get this very occasionally and I always pull out and try again 24 hours later which usually works.
To my uneducated mind this suggests the site is late renewing its certificate, but I will still avoid leapfrogging such a failure in the future - it can be caused by a rogue masquerading site I believe.