This table explains the data between eBaylink and Shopify


eBaylink Data   Shopify DataeBay Data
SKU    This is the Shopify product ID - a number generated by Shopify when you create a productSKU 
Retailer Reference    This is the Shopify Variant ID - a number generated by Shopify when you create a product
MPN    This is the Variant SKU from ShopifyMPN field in eBay
UPC
From the field labelled Barcode (ISBN, UPC, GTIN, etc.) in ShopifyUPC field in eBay