Because he wants you to pay for shipping and you might find the price more than you thought and he wants you to take the risk of attrition. Many fish die in shipping and if you ask for sensetive fish, he gets them, they die, and he is screwed - nothing to sell. You pay up front, it is your problem, not his.
I would NEVER do this but he is offering you a deal. Some of the fish he can get you will never see in your LFS.
George
George, Thank you - I could not have stated it better. Michael