Don't confuse postal regulations with NFA/BATF regulations. While it is allowable for 2 non-licensed residents to do a transaction, anyone in the business of selling firearms is required to hold a FFL. A company that offers firearms for sale on the internet is either a FFL or is doing so illegally. Either way accepting delivery through the mail is illegal unless you yourself hold an FFL.
"The whole of the Bill (of Rights) is a declaration of the right of the people at large or considered as individuals.... It establishes some rights of the individual as unalienable and which consequently, no majority has a right to deprive them of." (Albert Gallatin of the New York Historical Society, October 7, 1789)
"A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington