I would imagine there's been an a effort somewhere to try and make sellers aware of what their product is being used for. Either by authorities or by pro life people trying to "help."
Considering the legal and ethical implications of selling something you know or suspect someone is using to ctb then I can understand sellers freaking out about it.
In UK only:
Suppliers are supposed to report any suspicious attempts to purchase any of the class of poisons in which SN is included. The standard that they are supposed to apply is "reasonable grounds for suspicion of illicit use." See the Poisons Act 1972 (as amended). There were Deregulation Regulations promulgated in 2015 which are relevant too. Supplierss/dealers commit an offence if they do not comply, though I am not aware of any prosecutions as yet.
The thing is, what constitutes "reasonable grounds for suspicion ?" It's an entirely subjective standard which can only be decided by a court in each individual case, though our Common Law will allow a body of case law to develop, with the occasional blip both ways into perverse decisions. Most business owners will run a mile from any suggestion of non-compliance prosecutions, and will gold plate the rules to avoid any semblance of trouble.
On the other hand, you will find that there are some suppliers who for their own reasons will push the law to the limits of its tolerance for their own reasons, and will interpret "reasonable grounds" with extreme latitude. This isn't always for the profit motive, but can be what they see as a principled distaste for legal restrictions and Nanny State interference in individual freedoms.
It's noticeable that an online supplier in UK has just re-emerged. He seems to answer all the questions that you or I would raise.
Whilst I have no wish or need to purchase from him, I can see that it must seem to be too good to be true to some desperate souls.