It won't sell? Your brother bought it, didn't he? So, certainly it will sell!
If you decide to sell it on, then you are ethically beholding to believe that it is real. There is no reason - absolutely none - for you to believe that this is not an authentic item. Therefore, your hands are clean and you may sell this item as authentic.
However, should proof be furnished that this item is not authentic, then you'll have to (legally) take the item back and refund the entire purchase price. Nevertheless, you'll not be afoul of legalities, since you sold the item in "good faith."
Intent, man, it's all intent.
Mike.