Android Invoice Trading App

I'm looking for an experienced mobile app developer to create an invoice trading app for Android. The app should facilitate a trading marketplace and include payment processing. User authentication via email and password is also required.

Key requirements:
- Trading marketplace functionality
- Secure payment processing
- User authentication (email/password)

Ideal skills and experience:
- Proven experience in Android app development
- Expertise in building secure payment systems
- Knowledge of user authentication protocols

Please provide examples of similar work and a timeline for completion.