The first one ("It's your bag, isn't it?") is correct, because you're making a statement and then asking a "tag" question to verify that you're correct. The second example is already a question ("Is it your bag?") so you don't need to add another question.
Tag questions are used to confirm what we believe is true (e.g., "The meeting starts at 10:00, doesn't it?") or to involve others in a conversation (e.g., "It's a beautiful day today, isn't it?") or to ask their opinions (e.g., "I think he's very clever, don't you?"). However, the focus is on the first part of the sentence—what the speaker thinks is true—rather than the tag question, which is often just added to be polite.