public function whenBookSaleCompleted(BookSaleCompleted $event)
 {
     $book = $this->books->find((string) $event->bookId());
     $book = new BookTransaction($book->bookId(), $book->title(), $book->sellersId(), $book->sellersName(), $buyer->memberId(), $buyer->fullName(), $book->price(), true, false);
     $this->books->save($book);
 }
 public function whenBookSaleCompleted(BookSaleCompleted $event)
 {
     $this->books->remove($event->bookId());
 }