Apparently there are lots of fake headphones out there, with Audio Technica, Shure, Sony, Sennheiser and Bose being particularly vulnerable. However, even Bang & Olufsens have been faked. Here is a super website to check before you buy headphones over the internet. Don't think that this is an ebay problem. Fakes have been sold via Amazon as well.
http://fakeheadphones.com/
Here is an example of the detail you need to look for with some Senn CX300s
http://fakeheadphones.com/
Here is an example of the detail you need to look for with some Senn CX300s