2. The three ways to legally destroy a gun are: 1) destruction by the owner (google instructions), 2) giving the gun to someone who can legally possess it who will destroy it, 3) turn it in at a police station to be destroyed, although the guarantee depends here as some departments resell and won't necessarily tell you. You will need a federal license from the ATF to broker guns which is what your service will be doing despite your goals. You will probably need one organization per state since you can't easily trade guns across state lines. The military will absolutely not buy from you, even if you go to all the trouble to become a federal vendor.
Because of these difficulties, I suggest you stick to a simple pledge plus social media share system, provide destruction instructions and suggest gun-regulation lobbies to interested donors.