How do I keep inventory in sync across Shopify and other channels?

Pick one system as the single source of truth for inventory and sync everything to it, rather than updating each channel by hand. Shopify can be that hub for many sellers, with its sales channels (marketplaces, POS) tied back to the same inventory. For more complex setups, an inventory-management app or a lightweight ERP sits in the middle and pushes stock levels to every channel in near-real-time.

The failure mode is manual updates and lag — that's what causes overselling. Decide the source of truth, automate the sync, and set buffer/safety stock on fast-movers so a sync delay doesn't oversell you.