What is the ruling on working in large supermarkets where both lawful and unlawful items are sold, and is it permissible to work in a section where no unlawful items are sold?

Working in markets that sell both permissible and impermissible things is allowed if your work is in a section that does not sell prohibited items. However, it is better to leave such work and search for employment that is entirely permissible. You must also enjoin good and forbid evil to the best of your ability.