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.