Most relevant jurisdictions prohibit possession of controlled substances. Further, the legal words for facilitating the selling of Modafinil is conspiracy to possess a controlled substance with intent to distribute. (I don't endorse using legal jargon to scare people into doing what one wants, but I'm neither a prosecutor nor someone who could affect the big scary name).

Having looked at gwern's linked legal research, I suspect he's mostly right about the legal risks in practice for small orders.