SKUs listed across multiple channels are kept in sync by using Channel Connections.
Channel Connections are used to keep your:
Items in sync
Prices in sync
Auto-import and auto-list from one marketplace to other(s)
For items to be linked across your selling channels connected with SellerChamp, the SKUs of those items need to be the same. If the SKU does not match, then you can alternatively use the alternate SKU feature to link the items across the marketplaces.