If a business can convince a person to exchange his money for a product
who should care but the two parties involved in the transaction?
In a purely economic sense this statement would only be true in an economy with unlimited resources. It doesn't really apply to a crowded small rocky planet circling a medium mass star. And then there are the moral questions. Should a 13 year child be able to sell their bodies to adults for sexual purposes? It only effects the two parties, right? If one dictator wanted to sell weaponized nerve gas to another dictator, is it still viewed as a closed, private party transaction? Why can't cocaine and other narcotics simply be sold on street corners? If someone wants to buy it and someone wants to sell it, it should be okay with everybody else?
We all have a stake in what each other does. It's the basis of family, community, nationality and ultimately humanity. No man's an island.