I sell talleisim online. I certainly don’t encourage women to buy from me, but I no longer refuse, despite the Rema, since the famous tshuva by Rav Moshe Feinstein zt”l leaves room to be matir, and obviously it’s beyond my ability to assess her earnestness in observing the mitzvah.
But it seems to me that a yarmulke on a woman is more problematic. What if a I receive an order for a kippah and discover that it definitely will be worn by a woman (typically at a Reform bas mitzvah, etc.)? Is there any room to be matir in such a case?
It is preferable to avoid the sale, but hard to say that it is fully prohibited.
The reason for this is that even if a girl wishes to wear a yarmulke, it is possible that she will wear it in a “girly manner” (perhaps with ribbons or something) so that the prohibition of “lo tilbash” will not be transgressed.
It is also not clear that the prohibition of lifnei iver will apply, because the buyer can buy the kippah elsewhere. If only Jewish stores sell kippot, the transgression of lifnei iver will be contingent on a dispute among rishonim; if it is equally available from non-Jewish stores, there will not be lifnei iver in selling the kippah.
Also, refusing the sale won’t really help, because the item can simply be re-ordered by under some male identity, and it is hardly possible to filter customers on an online store.
[Concerning a tallis, see Iggros Moshe 4:49, where he writes that a woman wearing a “male tallis” (a regular tallis) would be transgressing “lo tilbash.”]