This may come as a shock to some of you but eBay does have an official policy on the sale of atomic / nuclear weapons. Yes, it seems the World is a safer place as eBay categorically states no atom or nuclear weapons are to be sold via eBay.
http://pages.ebay.co.uk/help/policies/firearms-weapons-knives.html
No Nukes on eBay
Along with the Non-Proliferation of Nuclear Weapons Treaty of 1968 I imagine this is one of the biggest factors preventing every crazed despot from acquiring their own nukes.