I think you severely overestimate the demand for suicidal substances versus how much more lucrative, from a dealer's perspective, it would be to sell substances that offer repeat customers. Also, storage of SN is a shorter shelf life and more temperamental than most drugs. Even KL, who was the "SN Kingpin" only sold about 1200 orders over the span of several years. That's nothing compared to what distributors of substances of abuse can make on DNM, a reputable vendor could easily sell that in a month-- bulk orders of expensive substances. "Making bank" needs to be a reliable income for these people to so severely break the law. It isn't just some occasional large purchases, it has to be enough to sustain a lifestyle. Likely the sellers of N like D and M and A, they were cartel members doing a small "side hustle" but even so, N is a drug of abuse and not just a suicide agent
Plus, most DNMs explicitly ban sales of poisons and there is genuinely no recreational reason to touch SN due to its lethality.
Basically: one of the only places on the Internet where SN is considered an important substance is this website. There is quite literally no reason to peddle it on a DNM , especially given the high profile case with KL right now.