There's a difference between "developing" a bomb and plain old purchasing one. Iran's working on making a bomb - is it too much of a stretch to think they've acquired bombs from Pakistan, North Korea, or the former USSR? I doubt alQaeda has the bomb - but Iran? I'd need proof to believe they don't have them...
Yes, it is far too much of a stretch.
Nuclear explosions leave a precise, unique radiation signature. We know which enrichment facility each Bomb tested by Russia was made. They know where we made fissionables for each of our tests.
It's like a fingerprint.
Selling a nuclear weapon would be the same as selling your own fingerprints to a known thief...you'd get the blame wherever "your" prints showed up.
In essence, you'd lose your own sovereignty; the buyer of your fingerprints could send you to jail at will by leaving your prints in obvious locations at crime scenes.