Fake high end shoes are BIG business on ebay. You have a high chance of getting ripped off. Of course the fakes are probably made of similar materials as real Nikes in sweatshops just down the street from the sweatshops that make legit Nikes, so how much you'd be getting ripped off would be debatable. Most people wouldn't be able to tell.
Try selling fake Nikes on eBay and see how far you get.
Contrary to popular online opinion, eBay (actually, the OEM) looks very closely at sales for fakes and will shut down an auction in a heartbeat if they believe you are selling knockoffs. All an OEM has to do is to notify eBay that an auction is selling fakes. eBay doesn't question their claim. The seller gets dinged, even if they weren't fakes, and multiple notices from eBay for selling fakes will get your seller account suspended rather quickly. There are some products that we won't sell on eBay because the OEMs will instantly report you, real or not, and we don't want the aggravation.
Regarding the OP, a lot of eBay sellers buy shoe stock from places that have gone out of business, shoes from last year, unclaimed stock (like you see occasionally on Storage Wars), and display models from highly national chain discount outlets.