A working showroom is a showroom in which people work. The one example I know of is in furniture companies, which furnish their offices with their own furniture, and use them as a showroom, rather than have a big showroom separate from the office staff. All the offices are set up by theme: the marketing office has all the trendy furniture, and the accounting office has a the traditional banker-esque furniture.

My sister works in a working showroom, and it's good because they get all the newest and best chairs, desks, and cabinets. The downside is, there are always people going on tour through the office, so the employees are required to keep their desks in pristine condition at all times.