Multi-Vendor Ecommerce Backend Development

I have a frontend ready for a multi-vendor ecommerce website, and I'm looking for a skilled developer to work on the backend and make some final adjustments to the frontend.

Backend Requirements:
- I need the developer to create a robust backend using Node.js and Express.js.
- The backend should handle user authentication and authorization, product and inventory management, and order processing.

Frontend Enhancements:
- The frontend needs some final touches mainly focused on UI/UX improvements, bug fixes, and performance optimization.
- Experience with React.js and Next.js is crucial for making these adjustments.

Third-Party Integrations:
- The backend should integrate with several third-party services including payment gateways, shipping services, and Email/SMS notifications.
- Previous experience with these integrations will be highly beneficial.

Ideal Skills:
- Proficient in React.js, Next.js, Node.js, and Express.js.
- Strong debugging skills and attention to UI/UX details.
- Experienced in integrating third-party services, particularly in an ecommerce context.