“At the very least the store owner should feel a responsibility to make sure that anyone who tries on clothing in her store should be allowed to buy it only if it conforms to the her own standards of tzniyus. How can I sell you something that goes against my own principles?”
DerechHamelech firstly, its not the store owners responsibility, and secondly everyone has their own level of tzniyus-it does not make any sense to force shoppers to conform to the owners personal opinion on whether something is tzniyus or not-its the shoppers. They will have their own din vecheshbon after 120-it has nothing to do with the owner.
I would like to add that many stores have a sign in the dressing room that say Halachos of tzniyus, as well as a mirror and a chair for that purpose.