There are tons of extremely complex and technical regulations governing the manufacture, sale, use, and possession of chemicals. Some lawyers spend their entire careers answering questions about them. The statutes that enact these regulatory schemes give pretty broad regulatory power to the various regulatory agencies. For example, the FDCA gives the FDA and the Attorney General broad power to regulate any "drug." And, for the purposes of the FDCA, 21 U.S.C. 321(g)(1) defines a "drug" to include any substance in the official United States Pharmacopoeia, official Homoeopathic Pharmacopoeia of the United States, or official National Formulary. These are three massive encyclopedia of chemical substances that includes almost anything you can think of. That statute is really old and was enacted when these encyclopedia weren't as massive as they are today, so there's a strong, certainly persuasive to me, argument that the original intent of the legislature wasn't to give such broad regulatory power to the FDA commissioner and the AG. But, would it carry the day before a textualist judge? The point is, the government has a ton of ways to regulate dangerous chemicals.
But I think the far, far more interesting question you raise is whether it's impossible to catch a murderer who uses this chemical. In short, I think justice could still find that killer.
The detectives are going to look for a motive, and find the people with a motive to kill. So that's going to narrow it way down. Once they've found those people, they're going to execute search warrants and subpoena the crap out of their purchase history.
Now let's say you've destroyed your computer, you made contact with the seller in person, and you paid cash. Even then, I imagine the half-life of the chemicals would allow the detectives to get a rough estimate of when the deceased was poisoned, and they could try to recreate the deceased's schedule around that time to find where she was, and who would have had access to her.
Finally, as to the point you raise about it being shocking that it might be easy to get away with murder. The sad truth is that it's not so shocking. I learned recently that clearance rate for murders by Chicago detectives is around 30%.