How do I keep stock in sync across multiple sales channels?

Use a system that centralizes inventory and updates every channel in near real time, so a sale on one marketplace immediately reduces availability everywhere else. Multichannel tools such as Cin7, Linnworks, and Extensiv Order Manager are built for this, usually through native connectors to your store and marketplaces. Syncing solves overselling, but it does not decide quantities. To avoid both stockouts and excess inventory, pair synchronization with demand forecasting that sizes inventory to each product's real variability.