Building a successful two-sided marketplace involves facilitating seamless communication between buyers and sellers, which is achieved through effective notification systems. The text discusses strategies to enhance user interactions and trust using Knock and Next.js, illustrated through a sample app called Globe Wander that connects travelers with tour operators. Notifications play a pivotal role in these interactions, initiated via various workflows that cater to different user roles, such as sending immediate confirmations or batching messages to avoid overwhelming users. The text highlights the importance of inventory updates, allowing users to subscribe to changes and receive notifications automatically, enhancing the user experience without creating a complex pub/sub system. Additionally, recurring notifications like a weekly digest can keep users engaged by providing personalized recommendations, fostering a healthy balance between supply and demand. Knock's features, such as workflow branching and scheduled workflows, simplify these processes, making it easier to build and maintain a thriving marketplace.