That is a myth, believe me. I have a degree in computer science and have worked in software development for a very big company and there is no way anyone, unless they are on the same subnet (and in eBay's case, that means in the same building), to find out your MAC address. It's simply impossible.
If you're on the same router, it's possible. But like I said, unless you're in the same building as eBay, you're separated by far more than just 1 router. And anything beyond 1 router, you're not able to see the MAC address.
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