What is the Sharee’ah ruling on selling sportswear to women?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

In principle, it is permissible to sell such clothes to women. However, if the seller knows that the women who buy such clothes will use them to display their beauty to non-mahram (permanently unmarriageable) men, then he is not allowed to sell them in this case as by doing this he helps them engage in sin and transgression.

The following was stated by The Permanent Committee for Islamic Research and Fataawa:

Everything that is used for forbidden purposes or of which it is thought that it is most likely used as such is prohibited to manufacture, import, sell, or promote among Muslims. This includes what many women do nowadays – may Allah guide them – of wearing transparent, tight, and short clothes. All such types of clothes entail displaying women's charms and adornments as well as outlining their bodily details before non-mahram men. Shaykhul-Islam Ibn Taymiyyah said, "All types of clothing that are most probably used for sinful purposes, it is not permissible to sell them to or sew it for one who will use it for sinful and wrongful purposes. If it seems most likely that garments will be used to help commit sin, then it is not permissible to sell them or sew them for one who will use them for sinful and wrongful purposes. Hence, it is disliked in the Sharee’ah to sell bread and meat to one whom you know will drink wine with these or to sell fragrant herbs to someone if it is known that he will use those for wine and immorality. The same applies to everything that is basically permissible if it is known that it will be used to commit a sin or help in this.'"

Allah knows best.