I'll create a mobile-first marketplace app called "당근마켓" (Carrot Market) for secondhand trading with photo uploads and chat functionality.

I've created a comprehensive mobile-first marketplace app called "당근마켓" (Carrot Market) with the following features:

Key Features:

1. Home Page (/)

   • Mobile-optimized grid layout for items
   • Search functionality
   • Location-based browsing
   • Bottom navigation with tabs
   • Floating action button for posting items

2. Item Detail Page (/item/[id])

   • Image gallery with swipe navigation
   • Detailed item information
   • Seller profile with ratings
   • Like/favorite functionality
   • Direct chat button

3. Chat Interface (/chat/[id])

   • Real-time messaging UI
   • Image sharing capability
   • Item context at the top
   • Mobile-optimized input area

4. Post Item Page (/post)

   • Photo upload with drag-and-drop
   • Category selection
   • Price input
   • Detailed description
   • Location settings

Mobile-First Design:

• Responsive grid layouts
• Touch-friendly buttons and interactions
• Sticky headers and bottom navigation
• Optimized for thumb navigation
• Korean language support
• Orange color scheme matching Carrot Market branding

The app includes all the core functionality of a secondhand marketplace: browsing items, viewing details, chatting with sellers, and posting new items. The design is clean, modern, and optimized for mobile use with intuitive navigation patterns.