I am a web publisher myself and I use Google Ads on my sites - publishers can definitely block the sites they dont want to apppear in their ads. I dont know about impressionz as I have not dealt with them. Bottom line is that someone chose to have the ebay ads on here - so money is being earned every time a visitor clicks away from here to go to ebay or other auction sites ............
The application used to block certain sites from appearing in the ads is called a Competitor Filter, its primary use is to block ads from your competitors from showing up on your site. Publishers also use it to block low quality or irrelevant sites too.