Well, my theory its that the guy who sells the jerseys have omenone who works in the Nike factory in Thailand (they have a factory there), so he gets the jerseys free...
Another theory its that he guy sells the jersys cheaper so he can make more sales, at the end he sales more than a person who sells the jersey in $ 45:
More people : More money
I also want to add that I have a friend that bougth a Juve jersey from subside and I compared to the one that I got in ebay and they are the same, in quality, texture, detalis, etc...