Presumably the problem only exists on specific computers. If it were an eBay problem, thousands of users would have been affected, and I'd imagine a lot more would have been said on these boards by now.
Remember a while ago various bits of adware seemed to sense specific shopping pages and only launched their ads when those pages were loaded? I'm wondering whether this might be something similar - a piece of software which lurks in wait for a particular page to appear, before hijacking it.
It's unlikely that eBay will be able to assist, although I suppose those affected could try their luck with Customer Servies.
It might be more rewarding to accept that this is going to take a while, and to try to locate and remove the problem on the affected computers. I'm bound to leave bits out, but I'd be inclined to proceed along these lines:
Check the computer is fully "patched" - with all Windows and third party appllcations up to date.
Take a close look at the Add/Remove Programs (or whatever it is called on your version of Windows), and see whether there is anything unusual there. If so, look any suspect programs up on the internet to see whether any of them is dodgy. Just be careful not to click on any installation pages you come across...
Toolbars are high on the list of Usual Suspects in this process. Frankly, I've never seen the point of the things, but even if you like them, it might be worth ditching any you find and seeing whether that helps. Any you really need (probably none) can always be reinstalled afterwards.
Find out how to clear ALL local data (cookies/cache) from your browsers, and do it.
If you have Flash Player, find out how to use its Settings Manager and clear everything it has stored. (You may need to reverify with a telephone numerical code next time you try to sell something. A minor inconvenience, and worth it if you get rid of something nasty.)
You're probably still not out of the woods. Run full scans with your antivirus, and with at least a couple of anti-scumware apps. I'd go for Malwarebytes and AdwCleaner. If you can face it, it would do no harm to run SuperAntiSpyware, too. There are one or two others mentioned on the pinned thread at the top of the board. (AdwCleaner will want a restart. It would probably do no harm to restart after you've completed everything else, too.)
Clear anything found during scans.
Manually check your browsers to see what add-ons are installed. Remove any you don't remember installing.
Reset your browser to its original state (it's easy to find instructions for this on the internet.) This will necessitate re-doing your usual customisations - disabling third party cookies, reinstalling wanted add-ons and so forth.
Many of these annoyances install themselves along with some piece of otherwise useful free software which we've installed, and it's quite possible that the problems folk are experiencing here might be related to other pieces of software which have been installed. On the increasingly rare occasions when I fire up a Windows computer now, I always run Malwarebytes and AdwCleaner after downloading and installing anything, even programs from reputable sites.
All a bit tedious but, without knowing just how dangerous your computer's infection is, it might be safest to take extreme action to get rid of it. It could just be an annoying piece of adware; it could be something much nastier, hoovering up goodness knows what information/logins/financial details etc etc.
