Actually if you look on Mophie's site, Amazon is listed as one of their authorized online resellers. So you can buy real Mophie cases at a better price on Amazon, but you need to buy it directly from them, NOT one of the third party resellers.
Most of the stuff on eBay is a knock-off. That is not to say they don't work. They may, but if you ever have an issue, Mophie will deny you any warranty support and then you are out money.