I wonder if the private merchants chose to purchase those firearms purely for profit or if part of it was to "stick it to the man", so to speak. They were likely people opposed to the buyback in the first place, and believed that the firearms were better in the hands of private companies and owners than in the hands of the government. To me, this is one of the cultural issues at hand in the US (my earlier comment about distrust of government or social systems).
The majority of Australians were opposed to the gun laws that were enacted, but the results have been very positive. There are other countries in the world where guns are a common part of daily life, but the amount of gun-related killings and accidents are significantly less than in the US.
A little of each I imagine. I once saw a vintage Martini-Henry rifle from the 19th century and a civil war musket at a gun buyback, both of which would go for multiple times the $300 gift cards the Seattle PD were offering.
And like I said, we have practical concerns that the Australians never had. I've yet to hear your answer to those.
I'd also like to point out that America's "gun problem" is not pervasive. I've lived in America my entire life and I've never once been shot or shot at. Hell I've never even seen a gun that I didn't want to see. The constant gun violence is largely isolated to the inner cities which, ironically, have the strongest gun control. The occasional mass shooter is random and gets a lot of media coverage, but you're still more likely to be struck by lightning than be attacked by one. It's a statistical fact that 99.997% of Americans will never be shot.
Other countries are typically much more culturally homogenous than America, and don't have things resembling American inner-city ghettos. Here's a fun statistic by the FBI: African Americans are responsible for half of all US crime despite being less than 1/5 of the population. No racism, just hard statistics from a reputable agency. I think that signals a problem with African American culture at large, personally.
Bottom line is the only real solution to gun crime in American is focusing on the fundamental causes of crime. Frankly, America is historically unique in every sense when it comes to guns. No modern nation has ever been so well armed for so long. No national population has ever treated the right to bear arms as we have. Very few nations have anything like our state concealed carry programs. Every international comparison is Apples to Eggplants.
Even Australia, which you argue is the closest, only had several million guns at its peak. We have over 300 million, and we aren't an island who's borders are naturally sealed.