For traditional retailers selling physical goods, SHELF SPACE is the biggest constraint limiting a firm's ability to offer customers what they want when they want it.
Let's say that you're opening a physical book retailer. The amount of books that you could put in store is very limited to the size of your store.
This type of problems wouldn't be faced by online-based book store, which could store unlimited amount of books because they do not need any space to store their goods