What is the ruling on a man selling women's underwear? And are there any regulations concerning that?

There is no impediment to a woman going out to buy her clothes, provided that she exits modestly dressed, not displaying her adornment, and interacts with the seller according to the regulations of Sharia, such as not speaking in a soft or alluring tone, and avoiding unlawful seclusion. However, it is better for her to remain in her home and delegate someone to make purchases for her, or to buy online, especially given the prevalence of temptations.