I'm going to move this to the NFA forum...
The store is both right and wrong. Supressors are perfectly legal if you do the proper paperwork and wait for ATF approval.
Certificate of Compliance:
Finger Print Cards:
You also need to find a Class 3 dealer to do the transfer, I'd guess that they probably have all these forms.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.