I'm not sure what you mean by "drugs to ctb". They don't sell SN on the dark web.
Drug laws vary widely country to country. If you live in a state in the US where marijuana is legal and you're buying small quantities of weed from someone living in that state, your risk is minimal.
If you live in China and you're buying 2 kilos of heroin from Columbia, your risk is bigger.
Each vendor has individual feedback (like Ebay). If some has high feedback rating, it's doubtful you'll get scammed.
It depends on what you're buying, but typically if law enforcement becomes aware of a package in the US mail containing illegal drugs, they simply confiscate the package. If you're buying something like cyanide or something that could be harmful to others, they might take more of interest in you.
If you're buying fentynal from California to get shipped to Montana, it's doubtful anyone would take much of an interest; but I'm sure the penalties could be steep.
Here are the federal penalties for trafficking:
(note Biden rescheduled marijuana so it's no longer considered "one of the most dangerous substances known to man with no medical value").