all brands have fakes out there. And even if you buy from a known store, there isn't a 100% guarantee that it will not be a fake (chances are less ofcourse). But very cheap bags new from e-bay should be bought with caution.
and I do not know how much gear you will be carrying, but the flipside400 is a rather big bag too. personally I would go for the flipside sport 15L. or rather even for the 20L because that has a padded waistband. and the 20L is the same size (bit smaller) than the flipside 400. the 15L is really smaller in dimension.
But have you also given though on f-stopgear?
Mountain - Products
Also back-opening systems, and you can personalize them to the amount of gear you plan to carry.