I had a chat with my local Computer expert and it seems we have Google to blame, not Ebay. All the ad space on Ebay is handled by Google Adsense and Ebay have no control over what appears there.
Adsense is the system that detects your browser history and tries to put up ads relevant to you. Often you get caught out clicking on an apparently empty area of screen and trigger the Ad that is still loading to that space - hence a pop-up or redirect to another webpage. Google do very little checking on who rents space through them, so it may be a genuine and innocuous ad, or, in the worst case, a site that is attempting to install malware on your computer. Hence why it is insane to go on the web without good antivirus and anti malware installed. Adguard and similar things will block 90% or more, but nothing is fool proof.
Perversely you will get les ads if you use Google Chrome instead of Microsoft IE as your browser - not sure if this is because Chrome is better at detecting malware sites, or if Google is deliberately down-rating the IE experience...
It's a jungle out there!