In point of fact, they tried and failed to get uranium.  But the issue here is not what is possible - its what is fair and just in society.

You accept that a legal system that prevents the materials for a nuke falling into the hands of a Jared Laughner or an Osama bin Ladin is needed.  I assume you agree that we need a legal system to monitor all purchasers of uranium and centrifuges so we know what they are up to.  And if we are not happy that the use if legitimate, you'd agree that we have to intervene before the bomb is made.

OK - then we are in agreement.  It sounds like you'd like to reproduce all the present systems regulation but use some new libertarian framework.  But as long as the regulation prevents things like nukes, smallpox virus, the huge amounts of fertiliser needed for bombs and the like falling into the hands or the mad and the bad, all is good.