You’re getting a lot wrong here.
“However, one needs a federal permit to sell one.”
Incorrect.
You only need an FFA if you are setting gun as a business venture. Basically selling more than a certain amount in a set period of time. Selling a single gun is permitted without any license in almost all states.
“Buying a gun from an unlicensed private party is called a “straw purchase” and is illegal”
Incorrect again.
A “Straw purchase” is one person buying a gun for someone else to actually posses, (Usually someone who is legally prohibited from owning a gun) thus allowing the receiver of the weapon to avoid paperwork and legal requirements.
“and the main source of weapons being used in crime.”
And again, Incorrect.
Stolen guns are used in crimes more often that any other method of acquiring a gun.
Thanks, Jotmo.