Well that is kinda solvable for making that completely illegal, and making the original owner an accomplice to any crime the buyer commits.
But I agree that no banning would stop a "regular" criminal, this is more about stopping mass shooting incidents no?
Only somewhat, with 3D printing one could print a lower receiver (where trigger and magazine go) and attach it to a standard upper receiver with any barrel and stock they want and go to town, sure they'd be limited to how many rounds that lower will sustain....but it's something you can throw away and attach another one.
As far as straw purchases go, for pistols there is a background check, however for rifles (depending on the state) no background check HAS to be performed, you could exchange a rifle between 2 individuals with just a written piece of paper that says "I, John Doe sell <rifle and serial number> to John Smith".
Making the original purchaser responsible is only good if the original serial number isn't removed, but you could also possibly be ruining a gun collectors life or reputation if you attempt to make them an accomplice because they sold a firearm to someone who does harm with it, it's up to the FFL to perform the background check as any rifle,shotgun, or pistol must be shipped from an FFL to an FFL where the receiving FFL performs the background on the individual.
There is no "good way" to solve this that will make everyone happy, on one hand we have school shootings taking lives, and at the other end we have the 2nd amendment. The bigger issue is look at the times where guns were taken away from the people either by the people giving them up willingly or they were taken forcibly, none of those times ended well for the people on the receiving end.