But there are severe penalties for dealing and using cocaine at all. And generally speaking, those that deal cocaine are not upstanding citizens. The relationship between dealer and user is considered in this country, and for all I know most every other country in the world, to be inimical to the common good.
Should we consider the relationship between those who transfer a firearm to be universally corrosive to the public good as well?
Analogies regarding firearms are always hampered by the nature of the object itself. Every aspect of any gun facilitates its use for great harm or great good, depending on which way it's pointed, who's doing the pointing and why. So anti gunners frequently compare guns to prohibited substances and pro gunners compare them to safety equipment. The gun is the same, but the relationship between "pointer" and "pointee" is an endlessly fluctuating thing that defies easy definition. The same holds true for transfers of guns. The gun doesn't change, but the relationship between transferees depends on near uncounted factors that are beyond the control of those wishing to regulate them.
That's why exceptions are already understood for close family members and the Manchin/Toomey legislation focused on gun shows and internet sales. The underlying criteria was the nature of the relationship of transferees. If people know each other well enough to know whether or not one of them should have a gun, then it's none of the governments business what they transfer between themselves. But if it is just a chance meeting at a public event or an internet hookup, the assumption is that someone is trying to acquire a firearm for nefarious purposes. Unfortunately, people who know each other can transfer firearms for nefarious purposes, and people who would never dream of breaking the law can meet and complete a transfer who may have only met for five minutes. The law was trying to posit the potential for misuse based on intimacy without regard for the intent of the transferees, because it's impossible to do that. The issue always goes back to due process.
If the state wants to regulate the transfer of an object that has no universally understood inherent danger to the public good, it has to evaluate that danger using the intent of those possessing the object. That evaluation is very problematic without a previous record of malfeasance and due process to deny the transfer. Given the number of guns in the United States that are never used at all, much less the number of guns that are used for the wrong reasons, the infrastructure required to evaluate the relationships of people who transfer them seems to me to be an unnecessary invasion of privacy with almost non existent remedy for that invasion.