Alice wants to order from an eshop abroad. The shipping prices are however twice as high as the price of the book. Alice goes to the Shipshare and publishes her intent to order from the eshop to her city.
Bob is in the exact same situation. He checks the Shipshare and finds Alice's order. He's not exactly from the same city, but it is much cheaper for him to travel there and pick the book at Alice's place.
Now that Alice and Bob are aware of each other's intent, they can order books together and save on the shipping prices. They might also decide to wait for someone else, to get a free shipping.