Read the following case study:

Shelf Traders is an online platform for exchanging used books. To offer books for sale, a seller must register with the Shelf Traders website. A seller must first register by providing their current physical address, phone number, and email address. Once registered, the system will create an account for the seller, which can be accessed through a secure and authenticated portal. Moreover, a seller can list books on the system by filling out a particular form. The form contains all the related information about the book: its category, description, general condition, and price. A seller may list as many books as desired. Moreover, the system maintains an index of all books, which enables buyers to look for specific books. In particular, the books can be searched by title, author, category, and keyword.

When buyers want to purchase books, they visit the Shelf Traders and search for the books they want. If they decide to buy, they must create an account with personal information and credit card information to complete the payment process. When a purchase is made on the website, an email notification containing the payment information is sent to the chosen book's seller. The book is then marked as sold in the system, and an open order is maintained until the seller confirms that the book has been shipped. The seller must make the shipment within $48$ hours after sending a confirmation email to the buyer. Once the shipment is made, the seller sends a notification to the buyer, and Shelf Traders maintains the order as shipped.

Based on the case study, develop the following UML models. If some information needs to be completed, make your own assumptions, clarify them in your answers, and reflect them in the diagrams:

b$.$ Choose one of the possible complex objects in the Shelf Traders system and identify its states that can pass through in the State Machine Diagram.