I can, however buying a propane tanker, or attempting to find out where one is going is going to attract far more attention than buying fertilizer.
My point: These people have had these guns, even while they made plans with poop. There was a reason they chose the poop. First it was their best chance at improvising a device to hit a specific target without being noticed, second a good-sized fertilizer bomb is far more efficient than any gun at all, anywhere.
Those who would buy these weapons have the knowledge to improvise those devices. They do so. Those who go nuts with guns don't have that type of knowledge and access to that kind of weapon.
So... The idea that we should use the "they may go crazy" reason to ban the weapon is what I object to. They may go crazy, so we need to ban poop, not this weapon.
There can be many reasons for banning it, but not the emotive "They might go crazy" reason. It isn't logical. When people who own that type of weapon go crazy they go for far more damage and for specific targets that provoked their particular insanity.