many places such as ebay don't allow counterfeit merchandise to be sold, and if you represent it as a real item you will open yourself up to a lot of complaints from customers who thought they were buying a real iphone... the other problem is that knockoffs are never as good as the original, nor do they work the same way. Sure, the case looks the same and the interface looks very similar, but that's as far as it goes. I bought a knockoff ipod nano a couple of years ago and I ended up giving it to a friend's kid because I wanted a real ipod.. the interface on the knockoff was NOTHING like a real ipod.
on a side note, I once saw fake ipods being sold at a flea market for like $50.. they were called ePods I think? was just the casing!! They didn't have anything inside them.