Could selling non-tznius clothing in a clothing store be considered mesaye le-dvar aveira?
As a limud zchus (a line of reasoning to judge others favorably for their actions) Rav Vosner, as I understand it, says it is possible that this doesn't fall under the category of assisting a transgression since it is possible that the clothes will be worn in such a way that they are not actually immodest (i.e. layered) or because one is not participating at the actual time of the transgression, but nevertheless a Ba'al Nefesh should avoid doing so(Shevet Ha’Levi 2:62)
See this article (PDF).
Yirmeyahu, great source. He points out in that responsa that wearing non-tzeniusdik clothing is prohibited min-HaTorah, for it causes men to sin.
It seems to me that one could argue that this consitutes "Mesaya D'Mesaya" i.e. assisting another to assist a third person to sin, which may not be a problem. See Avodah Zara (14a) and (21a) where it is stated that Lifnei D'Lifnei is not a problem. Certainly for mesaya, which is only M'Derabanan, it should not pose a problem either.
OTOH, one could argue that the Rabbinical prohibition of Mesaya does not distinguish between direct assistance or assisting through a middle man. Since no Misaya ever directly causes the victim to sin, and it is nevertheless prohibited, perhaps it is prohibited no matter how far removed from the actual sin.