Knock-offs abound on not only Amazon, but also on eBay. There was an article on HN recently about the difference between a knock-off Macbook charger and a real Apple one.

I'm one of those who unknowingly bought a knock-off charger off of eBay (the price was low so I bought it). I can't believe that they could make such an exact copy (externally). From the outside it's indistinguishable from my original Apple charger.