My Flutter app is almost ready for release; the missing piece is a direct connection to eBay. I need working, production-ready code that: • Authenticates users with eBay’s OAuth2 flow (sandbox and live). • Pulls their product listings through the relevant Sell APIs. • Keeps those listings in sync in real-time—using eBay’s Notification API, webhooks, or another proven strategy you recommend. I already have backend services that store user profiles and can hold the eBay tokens once you send them over REST, so your focus stays inside Flutter/Dart: network layer, data models, secure storage, token refresh, and solid error handling. Clear, well-commented code plus a short setup document will be the deliverable. If you have past experience with Flutter + eBay (or similar REST/OAuth integrations) let me know; I’d like to get this shipped quickly.