"The sitting man's bag is empty."
Translation:앉는 남자의 가방이 비어 있어요.
I like your sentence more. 앉는 남자 sounds like "the man who is currently in the process of sitting down"
이 identifies the topic whereas 은 is the subject. So 이 is mostly used the first time you talk about something. 은 is then used after or when the topic is established