I have the OB defender gray/pink I bought from my BestBuy at full price, and then I have 6 more that I bought from amazon. I honestly can't tell the difference between the BB one and the others I purchased at Amazon. So if the ones from Amazon are fakes, they sure are a good quality because I have no complaints about them
If your looking to only purchase one and stick with it and don't mind spending the bit extra, then purchase directly from OB's site or at a BestBuy. If your looking to get a few and mix the silicone outer and shell colors up like I do, then I'd suggest using Amazon.
Also, as Cleveland mentioned, check the sellers background before purchasing from Amazon, or if you decide to go the EBay route.