Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Men can work in such kind of shops as long as they, both men and women, keep modesty.
2- It is preferable to nominate women for such jobs, especially in small shops where there is close dealing and long conversations between the seller and the buyer.
3- Selling women’s underwear by men should either be done in large stores or it should be done in stores that are exclusively for women.
Responding to your question, we would like to cite the following fatwa published on islamqa.info:
There is nothing wrong with opening a shop to sell women’s underwear, so long as one pays attention to the Islamic rulings. So he should not sell anything but that which it is permissible for him to sell, which does not contain anything prohibited, like crosses, and he should not display these clothes on models in a way that may provoke desire or is contrary to modesty.
With regard to men working in such shops, in principle it is permissible for a man to deal with a non-mahram woman, buying and selling, renting, and so on, so long as the seller adheres to the Islamic guidelines.
One of the matters to which attention should be paid with regard to selling these kinds of garments is to differentiate between two business formats:
1- Large stores in which the goods are displayed and already priced, and the woman picks up whatever she wants, and the role of the seller is limited to working out what the customer owes only. There is nothing wrong with this format.
2- Small stores in which the woman needs to speak with the worker and ask about the characteristics of various products, and the seller needs to present the product attractively and promote it. So, the seller of women’s underwear may indulge in talking in a way that causes embarrassment or there is the fear of temptation, especially nowadays when dignity is lacking and many women have little modesty.
Such work is not befitting for the pious Muslim who is keen to adhere to his religion, and he should avoid it, because of what is narrated of Hadiths that warn against the temptation of women. That is supported by real-life stories of temptation experienced by many of those who work in such shops.
In a hadith the Prophet (peace and blessings be upon him) said, “I have not left behind me among the people any fitnah (temptation) that is more harmful to men than women.” (Al-Bukhari and Muslim)
Also, he (peace and blessings be upon him) said, “Beware of this world and beware of women, for the first fitnah (temptation) among the Children of Israel had to do with women.” (Muslim)
Hence, selling women’s underwear should either be done in large stores, as described in the first example, or it should be done in stores that are exclusively for women, in which women are in charge of selling things that are only for women.
And Allah knows best.