### Added
- **Sell from Holdings:** Players can now sell items directly from their virtual holdings without withdrawing to inventory first.
- **Market GUI:** Item details panel now displays "Inventory: X | Holdings: Y" and includes new "Sell 1 Holdings" and "Sell All Holdings" buttons.
- **Custom Shop GUI:** Vanilla items can now be sold from holdings when inventory is empty or insufficient.
- **Web API:** New `POST /api/sell-holdings` endpoint for selling holdings via web dashboard.
- **Command:** New `/market sellholdings <material> <amount>` command for direct holdings sales.