This table explains the data between eBayLINK and WooCommerce





Omnivore Data   WooCommerce DataMarketplace Data
SKU    This is the WooCommerce product ID - a number generated by WooCommerce when you create a product
Retailer Reference    This is the WooCommerce Variant ID - a number generated by WooCommerce when you create a product 
MPN    This is the Variant SKU from WooCommerceMPN Field in Marketplace
UPC / EAN / ISBN
This can be set up as as a custom attribute of the product.  The name of the attribute must be one of UPC, EAN or ISBN, as appropriate in WooCommerce. See FAQ UPCs, EANs in WooCommerceUPC field in Marketplace