I would like to mention how Microsoft, for example, tells the difference if you're using a different PC to install their software products on. Bear with me, for I'm trying to explain why I believe eBay won't easily be able to do the same. Say you install Windows on PC1. After it's been installed, you're asked to validate your copy of the software. Let's say, the validation is complete and you're set to use Windows. What happens if your Windows crashes? No problem, you can reinstall it and validate again. The validation routine won't complain. However, if you were to install this copy on a different PC, the validation won't go through. This validation routine actually looks at the hardware component makeup of the original computer you validate your copy on. It communicates that makeup to Microsoft's server and stores it. That's why if you signicantly upgrade your PC, thereby changing the hardware component makeup, even though it's still the "same" computer as far as you're concerned, the validation may fail.
Tracking by way of IP and cookies doesn't require any routines running on your PC to actually scan the underlying hardware and determine its makeup. Applets or other various browser objects can potentially do that, software applications can do that too. Javascript cannot. A scripting language just doesn't have the provision for that. But anything that can actively sit in memory and perform its task until you terminate this process has the potential to access the underlying hardware layer. While you have your browser running with a browser object or applet running as pretty much a browser plugin, it is in memory until you terminate the browser itself. So there is potential (if the capability has been embedded into the object) for being screwed. That's how malware works and that's how some viruses gain access to the windows registry... For ebay to get you like this, it could be done through any software you may allow to be installed. Paypal tool bar, Ebay tool bar, PP plugin or an actual ActiveX component that you've allowed to run on your computer. TurboLister is another example. Anything that's installable (from a technical point of view), with or without your knowledge. So avoid all of the above.
Other than that, it's basically those little email images, IP and cookies.
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