Yesterday I spotted a pair of counterfeit Sennheiser HD 598s on eBay, listed by a seller with 100% feedback through several hundred transactions.  I contacted the seller and alerted him to the situation.  I included a link to the AliExpress listing for the fakes, noting the most obvious differences between them and the real thing:
 
1) The ear cushions on real Sennheisers are covered in darker, denser, richer velour; on the fakes you can see the weave of the fabric through the scruffy nap.
2) The velour on real Sennheisers tucks right into the earcup; on the fakes it's crudely sewn to a piece of beige vinyl that peeks out between the plastic earcup and the velour.
3) The pad across the headband is stitched on real Sennheisers; on the fakes it's just molded.
4) Inside the headband above each earcup, real Sennheisers have four screws; the fakes have three.
At least with the HD 598s, the cheaters made easy work of spotting the fakes!
 
I'm glad to say that the seller immediately took down the listing and thanked me for catching the problem; he had no idea these were counterfeit.  Better still, he thinks he can get a partial refund from his vendor.
 
It's been said before, but it bears repeating: if you buy popular headphones from any but an authorized dealer (even an honest and reputable vendor), you may get stung.  Decide for yourself if the saving is worth the risk. If you buy online, do your homework and shop smart!

Head-fi is aware of the hd598 been counterfeited. 

Please post other fakes headphones you find in order to improve the list and educate customers. I remind everyone to buy from authorized sources if they want to have warranty.  
http://www.head-fi.org/t/691594/counterfeited-headphones-list

Yes, I saw the HD 598 on that list, thanks.  In fact, that's what rescued me from bidding on this pair of 'phones.  I posted for three reasons:
 

• I wanted to highlight the obvious differences between the real and the fake.  If you know what to look for, one glance at a decent photo will tell the story.
• Even honest vendors sometimes (unwittingly) sell counterfeit merchandise.  This underscores the value of buying new from an authorized dealer.
• A few years ago, threads on head-fi helped me find a pair of genuine Sennheiser CX-300s.  This forum has saved me twice now, and I wanted to give something back.

 
Of course, your point about the warranty is also very well taken; I neglected to mention that. 

I think amazon is safer than ebay for selling genuine products. Any other good online stores that sell genuine Sennheisers?