Is selling wedding dresses and trousers that might be worn by non-hijabis, and "fashionable" clothes that might be displayed in mixed-gender halls, considered spreading indecency and a sin upon the shop owner?

Selling clothes is not considered spreading indecency or assisting in it, and it is permissible if the Islamic legal guidelines are observed. The sin falls upon the one who wears them and flaunts herself in them, and upon her guardian, not upon the seller, unless the seller knows that the buyer will flaunt herself in them. In that case, it is not permissible for him to sell them to her, because he would then be assisting her in sin. Allah Almighty says: (And do not cooperate in sin and aggression).