Nuclear materials are VERY carefully controlled, because ordinary citizens are not allowed to possess them. It's impossible to buy a nuclear weapon on the open market, as of right now.
However, if ordinary citizens WERE allowed to posses/build/buy nuclear weapons, it stands to reason that there would be nukes for sale in the open. And once that happens, ANYONE can buy one. Legally?
Do you honestly think that if nuclear weapons were legally for sale in the US, terrorist types would NOT buy and use them? The only thing that's stopping them is that nuclear weapons are difficult to procure ANYWHERE in the world.. and extremely difficult to sneak into the United States. I promise you, Al Qaede (or any of hundreds of other terrorist organizations) would love to detonate a nuke in the US, if they had the capability.
How would the country be better served if we ALLOWED them to?