Allaah has commanded the believers to cooperate amongst themselves in righteousness and piety, and He has forbidden them to cooperate in sin and transgression. He said (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression”
According to the shar’i principle set out by the scholars: “The means are subject to the same ruling as the ends.” So everything that leads to haraam is also haraam.
Selling haraam clothing such as short skirts and tight pants etc in which women go out is undoubtedly haraam because it leads to forbidden wanton display of their charms, by means of which women tempt men.
Those who buy these clothes usually go out and appear in them in front of people, so selling them is helping in sin and transgression, and spreading immorality and evil among the people.
Shaykh al-Islam Ibn Taymiyah said:
All clothes which one thinks most likely will be used for sinful purposes, it is not permissible to sell them or sew them for one who will use them for sinful and wrongful purposes.
Sharh al-‘Umdah, 4/386.
Even if the clothes are being exported and sold to kaafir women, it makes no difference to the ruling, because the minor rulings of sharee’ah are also addressed to the kaafirs, according to the correct scholarly view. This is the view of the majority of scholars. So everything that is obligatory for Muslims is also obligatory for kaafirs, and everything that is forbidden to Muslims is also forbidden to kaafirs. They will be brought to account for that on the Day of Resurrection, and their torment will be increased.
The evidence for that is the verse in which Allaah says of the people of Hell (interpretation of the meaning):
“’What has caused you to enter Hell?’
43. They will say: ‘We were not of those who used to offer the Salaah (prayers),
44. Nor we used to feed Al‑Miskeen (the poor);
45. And we used to talk falsehood (all that which Allaah hated) with vain talkers.
46. And we used to belie the Day of Recompense’”
The point here is that Allaah tells us that He will punish them on the Day of Resurrection, and that their punishment will be Hell, and that when they are asked why they deserve this punishment, they will say that it is for the sins of not praying and not feeding the poor. This indicates that they will be punished for that as well as for their being kuffaar who did not believe in the Day of Reckoning.
Selling these garments to those who you think will most likely use them for sinful purposes is haraam, whether those who buy them are Muslims or kaafirs.
And Allaah knows best.
See also the answer to question no. 34674. In the answer to question no. 3011 you will find an important discussion of a hadeeth from which it may be understood that it is permissible to sell things that are forbidden to Muslims if they are being sold to kaafirs.
And Allaah knows best.