Integrate inventory and order sync with Depop's private Partner Selling API

Steps

1. Request access from Depop partnerships — the Selling API is gated and not open self-serve signup
2. Once approved, authenticate with a static per-shop API key, or OAuth2 for multi-seller cross-listing tools
3. Test inventory create/update/delete calls in the provided sandbox environment before touching production
4. Create, update, and delete inventory items through the documented Inventory endpoints
5. Register a webhook subscription for real-time order and offer events instead of polling