Short answer: Multi-channel commerce becomes difficult when the same SKU, inventory position or order is represented differently across Shopify and marketplaces. A stable operating model starts by defining which system owns each piece of information, how products are matched, and what should happen when one channel changes.
Define the source of truth
Decide where product identity, inventory and order state are owned before trying to synchronize everything in both directions.
Match products and variants carefully
SKU and variant mappings should be reviewed before automation. Incorrect mappings can create overselling or update the wrong listing.
Separate inventory from order workflows
Inventory synchronization and order fulfillment are related, but they have different failure modes and should be monitored separately.
Handle exceptions visibly
Orders, listings or inventory updates that cannot be matched should become visible exceptions rather than silently disappearing.
Verify changes on each channel
After a sync or fulfillment action, confirm the resulting state on the destination channel instead of assuming the update succeeded.