(in the US) So that the government can conduct a background check on you to determine if you are legally qualified to purchase a firearm.
A private individual does not need a license of any sort to purchase a firearm from another private individual under federal regulations administered by BATF&E. State laws will vary, with some states requiring a permit to purchase a handgun.
Only when a private individual purchases a firearm from a Federally Licensed Firearm's dealer, is the dealer required to conduct a background check, called a NICs check, to verify that there is no record of the individual being legally prohibited from possessing a firearm.
No, you do not need a gun license to buy magazines.
yes
No it's not necessary.
No.
no
no
Yes, you do not need a gun license to buy a clip, but you may need one to legally possess and use it depending on local laws.
Generally, no.
yes, you just need a license
No ... but their may be an age requirement.
Yes.
No.