Started getting pop ups on Safari and ebay stating Safari can't verify the identity of the website "s7.addthis.com" and another saying the same about Stags.Bluekai. I've no idea what these are and why I'm getting them. I've emptied caches .

Thank you for your reply. I've always had pop up windows blocked and it still is. I did notice that Google wasn't working to block dangerous web sites and hadn't updated in 10 days so I restarted my computer in safe mode and when I opened it up in normal mode that warning has gone and it appears to be working now. I'm old and don't know much about Macs only what I've read on here and googled. I don't have any extensions it is switched off and always has been. The weird thing is that since I restarted my Macbook after putting it in safe mode... fingers crossed I haven't had either of these pop ups. I really don't ever download anything from anywhere other than the app store. My feeling is that something has tried to sneak onto my computer from ebay and my computer was constantly blocking it and warning me. If they return I shall follow your instructions. Thank you for taking the time to reply I really appreciate it.