Is it permissible to work as an intermediary for foreign websites to order clothes that violate Sharia (containing nudity or images of living beings), under the pretext that I am not ordering them for myself but for customers, while advising them that it is impermissible, yet they insist on buying them?

The basic principle is that trading and brokering in clothes are permissible. However, if it is highly probable that the buyer will wear them in a forbidden manner, or if the clothes contain forbidden images, then dealing in them is prohibited, as it falls under aiding in sin and transgression. What you mentioned is not sufficient to permit it. But if such a strong probability does not exist, then there is no harm.