Kate of the "That's So Fake!" blog has a great video on YouTube to help you determine if the OtterBox case is authentic:
You can also see tips about spotting fake OtterBoxes on Kate's blog here. If you come across a fake Otterbox, take a few moments to report it to the Otterbox Fraud Investigation Team.
How Vero Protects Online Sellers
Counterfeit Items are Not Allowed on eBay
Thrift Store Items to Sell on eBay for Profit