Cross-Platform Marketplace App Development

I'm seeking an experienced mobile app developer to create a marketplace app for buying and selling physical goods. The app needs to be available on both iOS and Android platforms.

Essential functionalities include:
- User Authentication: Secure sign-up and login.
- In-App Payments: Seamless, secure transactions.
- Push Notifications: Alerts for new listings, messages, and updates.

Ideal skills and experience:
- Proven mobile app development expertise for both iOS and Android.
- Strong background in implementing secure payment systems.
- Experience with user authentication and push notification services.
- UI/UX design skills for an intuitive user experience.

Please provide a portfolio showcasing similar projects.