That is in fact how buyback schemes work in countries that aren't totally irrational about guns.
Also America isn’t land locked like Australia. The black market will have an easier time transporting the goods through South America. If that wasn’t the case, then drugs wouldn’t be as accessible here. And given US is the arms capital of the world (American manufacturers supply a significant amount of the arms used in conflicts), those guns will inevitably find their way back to America (conflict stops, owners need money more than weapons, etc). The bigger the conflict the cheaper the guns become.
Finally, Australia had a national moment of unity when they banned guns and so giving up your guns was a collective activity. That’s not the case in America.