Product Catalog is the first place where you start listing your Shopify products on eBay or Etsy. As mentioned before, the identical products will automatically link by SKU and this will be showcased in the Product Catalog:

Here, you get an overview of your Shopify products, how they're connected to products in existing marketplaces (eBay or Etsy), and the ones that haven't been listed anywhere yet.

If you have identical items that do not share SKUs, you can link them manually. To start with, go to the integration page, in our example it's Etsy:

Here you can see that one of the products is already linked because it has the same SKU as the Shopify item:

Meanwhile, another product is not linked:

By clicking the red chain icon, you will be prompted with the manual linker window:

Here you can select the required item, and click the link button to link your products. Once that's done, you will see a green chain icon:
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The next step is to list products from Shopify to your eBay or Etsy marketplaces. Please read more information about this in the following article: Step 4: List your Shopify products.