The above commenter was saying that there is no way to find a user's MAC address by looking at their IP packets alone. MAC addresses are used by link layer protocols (ethernet), but an IP address is used for identification in higher levels. In short, your router knows your MAC address, but Amazon.com only knows your IP. If any website on the internet could get your MAC address that would be very troubling, because a well-behaved MAC address is a quasi-unique identifier for your network hardware.

This is only possible if there is software installed on the machine that is collecting the MAC address and sending it (browsers don't do this). You are likely confusing a MAC address with a unique (random) identifier assigned to your browser by Amazon.

Dude, it is the mac address they grab.

I ran ifconfig en1 | grep ether and the mac address showed up on my URL omnibar when I firebugged it.

I've even noticed this happening at Starbucks whenever you use their WiFi - they will sniff your MAC address and it shows up in the URL omnibar when you agree to the WiFi terms.

Will the real hackers please chime in on how this (grabbing MAC address) can be achieved...

Yes, this happens at Starbucks because your MAC address is captured when you connect to their wifi network (this happens on virtually any captive portal, not hard). I, for one, cannot find my MAC address in a long list of history because it is simply not passed via web requests.

You can get the MAC address of anyone who is connected to the same network - but that's it.

Amazon just needs to sustain a cookie in between times that they do have a chance to collect your MAC address. I wouldn't be surprised if certain plugins could detect it, like flash or amazon streaming plugins. Also, kindle on your home network could grab it.

I don't think Flash has that ability, but Java can if you accept the numerous prompts you'll get...