What is the ruling on selling non-Islamic men's clothing, such as trousers, and what is the ruling on working in this field?

Trousers are permissible for men as long as they are not tight or transparent, describing the 'awrah. They are also not exclusive clothing of disbelievers. Therefore, labeling them as un-Islamic is out of place. It is permissible to work in their sale and purchase unless they are of the forbidden description.