Is it permissible to advertise an foreign online store that sells mostly immodest clothes, knowing that the overwhelming majority of those who buy them are immodestly dressed women, even if it is not impossible for a veiled Muslim woman to buy them to adorn herself for her husband?

It is permissible to advertise and promote goods that are permissible to sell, and it is not permissible to advertise or promote what is not permissible to sell. Revealing clothes (thiyab al-tabarruj) are not permissible to sell to those who would disobey Allah with them, unless they are used in a lawful way, such as for adornment for one's husband. If the overwhelming majority of those who buy these clothes are women who display themselves immodestly (mutabarrijat), then it is not permissible to promote or advertise them.