Break the sentence like this and read to understand There is always hope== that== in its labyrinth of musty,dark,disordered rooms=== a real rarity will be found amongst the piles of assorted junk=== that litter the floors. If still not understood, place a comment, I will try to explain. :)

The sentence needs context. Only after reading it in context can I say that the sentence is discussing why a person will enjoy visiting an old antique shop. We can not say what "it's" refers to towards the beginning of the sentence without the full context. "In it's labyrinth" is saying "In the antique shops maze of stuff." A labyrinth is a huge maze that you walk through. The full sentence means, People think they will find a treasure in the maze of an antique store.

Well, try to catch the main idea of the sentence. There is a hope (what hope?) That a real rarity will be found.(Where it will be found?) Amongst the piles of junk. In the labirinth of rooms .