Could selling non-tznius clothing in a clothing store be considered mesaye le-dvar aveira?
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.
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).