 Beacons are specific to each browser...
so if you have two browsers, like I do, Explorer and Firefox, they each store and act on cookies in independent directories and are totally unaware of each other's cookies. So, a cookie stored in Firefox could never be accessed through Explorer and visa versa. But that also means when you clean up your cookies on Firefox you are NOT cleaning up your cookies in Explorer.
Beacons aren't a theory, they are a fact. All internet marketers use them. I just checked paypal, and they have a beacon from doubleclick. Surprise. But fortunately they do not have one from EBAY.
I do not think them directly connected, at least not yet. Otherwise, EBAY could just can your account the minute they got your email address for payment on paypal. I believe there are laws pertaining to privacy involved there. You have to ALLOW paypal to hand over your information and that is what the linking of accounts is all about.
That is why EBAY provides incentives to link your accounts. To catch you off guard seeking a bargain. Notice how they also offer a discount when you open account for free access to your bank account. If you give it, watch out.
Beacons are typically very small invisible icons. They aren't typically invisible and small to fool you. Shoot, you could be looking at a full screen image and not know it was a beacon. So why bother to use small invisible icons?
They are small because they render almost instantaneously. They are the first bitmaps even on dial-up links to render and each one can result in a cookie being read or placed on your system before you even see the web page you are about to browse or the email you are about to read.
Again, EBAY definitely uses them. They are most obvious in their generic messages, like the EBAY Welcome or the Congrutulations messages sent to all new users. How many people have you seen say, gee, I got suspended right after I got my welcome message. If you have gmail, which blocks them, you can see them as very small images along the sides or bottom of the message. If you don't use a mailer that blocks them, you don't see them at all.
Anyway, back to the browser question. It is possible to be using Firefox and clean up all your cookies and then open up a message using Outlook Express which uses Explorer. Even though your Firefox express cookies were cleaned up, outlook express opens an explorer window internally and exposes your system to all the beacons within. Lovely. If you have explorer cookies about, those beacons access those cookies and if they are EBAY cookies from a prior login, they know exactly who you are.
Outlook also blocks images for the same reason. Rule. Absolute rule no doubt. NEVER ever allow images to be displayed in any message received from EBAY.  | 
