copied a pasted from another thread 
Update your antivirus and run a full scan - remove any nasties;
Install, update and run Malwarebytes free edition - at least a quick scan - full scan if that finds nothing. Remove any nasties.
Install, update and run AdwCleaner - remove any nasties found (usually necessitates a restart.)
It might be worth seeking expert advice, or following the advice on a site like bleepingcomputer.com to fully rid your machine of problems.
http://www.bleepingcomputer.com/download/adwcleaner/
http://malwaretips.com/blogs/how-to-scan-with-malwarebytes-anti-malware-2-0/
These will give more information.
Unlikely to be directly eBay-related - some adware nasties only seem to work on online shopping sites.
All the usual suggestions apply - make sure you're using a limited user account, ensure that your operating system is fully patched, ensure that all third-party software is fully patched, and so on.
I'm not sure you can stop it, to be honest, when you sign into eBay, it says at the top that the site uses cookies and that by using ebay; you agree to that. One of the purposes of cookies, is to log the way you shop and give eBay inside knowledge ( so they think ) of your shopping habits and your likes and dislikes. I agree though, it is annoying, if I just happen to look at a pair of Socks; it doesn't mean I want Socks shoved down my throat for the next week.
