If I remember correctly one should always if using a Windows system, first ensure that you have removed all viruses and, or programs that are allowing the adverts to appear in the first place, before adding an AdBlocker.
There is nothing eBay can do about the programs you have installed on your machine or in your browser, that you installed inadvertently when downloading something else, if that is the cause of your problems. And it could very well be. Apparently.
The 'Pinned Thread' at the top of this sub-forum explains these things in the necessary great detail. Links to the programs needed to assist there might be outdated, though, they are still available elsewhere.
Malwarebytes is annoyingly difficult to configure in free mode, though, it is still freely available and even if you inadvertently choose the free trail version, you can use it and when the free trial period runs out, it will, I believe revert to the free version. You can't be charged if you don't give any financial information anyway.
It is still freely available and once installed a slow navigation of the possibilities will enable anyone to configure it onto it's free mode.
Another source of pop-up adverts is browser extensions that have been added without the users knowledge. Search the Internet for assistance. I only know Firefox and can assist there.
Good luck!
I've not seen an advert on eBay, youtube or any mainstream site for going on a decade, though, fortunately I don't use Windows systems either.
