This table explains the data between eBayLINK and BigCommerce.

eBay LINK Data   BigCommerce DataMarketplace Data
SKU    This is the BigCommerce product ID - a number generated by BigCommerce when you create a product
Click here to find how to find your Product ID in BigCommerce

Retailer Reference    This is a unique value constructed by eBay Link from Bigcommerce data to provide a unique value for each product variant in case of multi variant products
MPN    This is the value of the MPN that has been entered into the optional Manufacturer Part Number field within a product in BigCommerce. If there is no data in the MPN field in BigCommerce eBay Link will use the SKU value from BigCommerceMPN Field in Marketplace
This can be entered into the optional Product UPC/EAN field within a product in BigCommerce
UPC field in Marketplace

Please Note:

This is important information to note when creating Product Groups in eBay LINK.  Product Groups require the SKU we have in eBay LINK.  Not the sku you may have used in BigCommerce.

The actual field in BigCommerce called SKU is not a mandatory field nor a required field, so it is not a reliable field in BigCommerce.  Our App must have a SKU as part of the eBay listing requirement and because we cannot rely on BigCommerce providing that data via the SKU field we use the BigCommerce Product ID to create a SKU in eBay LINK.  This eBay LINK SKU cannot be edited and it must be used when creating product groups and promotions etc.