Edited by tuanh714 - 9/29/14 at 7:15pm
I don't know what you're looking into buying. But I've bought a few (headphone related) things from eBay, four pairs of used headphones actually. None turned out to be a problem with me. What I do is look at the sellers ratings and reviews, and see if they seem to be a legit seller. Anything that's too far below 100 percent I shy away from. And honestly, I'm not sure if I'd be willing to dump a ton of money into something on eBay unless it's something I couldn't find anywhere else and I really wanted it. For the most part, the things I've bought on eBay were relatively cheap (for headphone gear anyway). If you're worried about buying a new product and getting a counterfeit, then I'd just try and find an authorized online dealer. When I got my K240 Studio, the seller was AudioTopia, and on AKG's website they listed them as an authorized dealer, so I felt better.
Anyway, just some of my experiences and such.