But your argument also justifies selling arms to North Korea. After all, it's a positive-sum transaction that increases global GDP? The flaw is that a transact

But your argument also justifies selling arms to North Korea. After all, it's a positive-sum transaction that increases global GDP? The flaw is that a transaction isn't positive-sum just because you and your trade partner are both happy. Your transaction may well incur negative externalities that outweigh the total profit enjoyed by you and your partner. For example, the world is much worse off if you sell a button-that-destroys-America to North Korea, even though you and North Korea are both pleased at the arrangement. I don't want to put activity out of the reach of governments. Governments are the organ by which we decide what is and isn't acceptable behavior. If our governments are behaving unreasonably, the solution is to fix them, not to pursue anarchy.