Growing a two-sided marketplace presents challenges, particularly in maintaining buyer and seller engagement, which is crucial for sustainable growth. Sequoia Capital emphasizes that engagement drives retention and growth, and highlights the importance of using in-app notifications to enhance user experience and facilitate communication. These notifications, such as inboxes, toast messages, and modals, can effectively replace or supplement traditional email alerts by providing real-time updates and interactions. They help in various scenarios, including buyer-seller communication, reducing cart abandonment, preventing fraud, managing inventory, completing user onboarding, alerting users to price changes, and encouraging timely reviews. Successful marketplaces like Bounce and Discogs have leveraged in-app notifications to significantly scale their operations. By utilizing in-app notifications, marketplaces can drive engagement and sales while reducing reliance on email, ultimately creating a more interactive and secure environment for users.