Serbian Used-Items Marketplace App

I want to launch a two-sided marketplace where people in Serbia can buy and sell used goods directly from their phones. At first release I only need two category groups live—“Electronics & Home Items” and “Clothing, Baby Items & Sports Gear”—so the database, filters, and navigation can stay lean while we test traction.

Core user flow
• Account creation through either a classic email-and-password form or one-tap Google, Apple, and Facebook sign-ins.
• Post an item with photos, price, description, and location (GPS or manual entry).
• Browse, search, and filter by the two launch categories.
• In-app messaging between buyer and seller.
• Safe meet-up tips page and simple report/block tools.

Tech expectations
I need a single code-base that ships to both the App Store and Google Play. React Native or Flutter is fine—convince me which you prefer. Cloud functions, Firebase/Firestore, or a comparable real-time backend will keep iterations fast, but I am open to alternatives as long as push notifications and image storage remain smooth.

Deliverables
1. iOS and Android builds ready for TestFlight / internal track.
2. Lightweight admin panel to moderate listings and flagged chats.
3. Git repo with well-commented code and a README covering environment setup and store submission steps.
4. Two short Loom videos: one walking through the user experience, the other explaining the repo structure.

Acceptance criteria
• Successful sign-up/sign-in with each auth method.
• Item can be listed, discovered through search, and purchased via in-app chat.
• Serbian language strings externalised for easy translation updates.
• No critical crashes on iOS 15+ and Android 11+.

If you have live marketplace apps in your portfolio, please share the store links when you respond.