Are you certain those ads are actually from eBay? What is being advertised?
I do not think eBay uses pop-up ads, though eBay does use banner ads. At least eBay has not used pop-up ads in the past, and I do not recall seeing one.
I typically use an ad blocker and a pop-up blocker, so I suppose it is possible that eBay has recently started using such ads and I have just not noticed, but I suspect you may be seeing pop-ups due to adware or malware on your computer, or due to browser hijacking or something like that.
The shadier the actual ads you see, the more likely it is the result of malware, I think.
