I am launching a cross-platform marketplace that mirrors the smooth shopping experience people associate with Amazon. My priority is to have one coherent solution running on the web plus native iOS and Android apps, all backed by a single, well-documented API. Key technical notes • I am flexible on the stack—go with what you know best, provided it scales and keeps the codebase clean. • Checkout must route through UPI. If you prefer integrating UPI via Razorpay, PhonePe, Paytm, or another compliant gateway, outline that approach up front and include full callback handling and settlement confirmation in your build. • A lightweight admin area for catalogue and order oversight is needed; feel free to leverage an off-the-shelf template if that accelerates delivery. Deliverables I will review for sign-off 1. Production-ready web front-end deployed to a test URL. 2. iOS build (IPA) and Android build (AAB/APK) ready for TestFlight and Play Console. 3. Source code for front-end, mobile apps and back-end (Git repo). 4. Database schema plus any seed/sample data. 5. Payment flow demo video showing a complete UPI transaction and successful callback. 6. Setup notes so I can reproduce the environment locally and on my cloud account. Demonstrate previous marketplace or payment-heavy projects if you can—seeing a live link or repo snippet will speed up selection.