If you are approved for a UPC exemption on Amazon and/or Walmart, or don't have UPCs for your eBay listings, Zentail allows you easily create and sync UPC Exempt listings on these channels.  Note: for some channels and categories Brand and MPN is a sufficient substitute for a UPC.  As a best practice for a multichannel business, Zentail recommends having a UPC for all your products and unique UPCs for your kits.  
This article covers three topics:

Simply set the "Upc Exemption" field to "part" for single products, or "bundle" for kits and multipacks, then set the product to list!  Zentail will automatically handle both Amazon's and  Walmart's unique requirements and send "Does Not Apply" to eBay. You will need to have a Brand, Manufacturer and Model Number (for Amazon) filled in to successfully create the listing.  Setting the UPC Exemption field can be done via Import, Bulk Action, Quick Edit, or under Edit > General Info.

Note:  If a UPC or Desired ASIN is filled in, that will be sent instead of the UPC exempt listing components.

Just set the SKU to list!  Zentail will automatically check for a listing with a matching SKU value and connect to it.  Note:  It is recommended that once a UPC exempt listing is created, you use the identifier from the channel instead of continuing to have it set as UPC exempt.  For instance, use the ASIN on Amazon as the Desired ASIN in Zentail, or use the GTIN in Walmart as the UPC in Zentail.

Amazon manages this by brand, and Walmart manages this by brand and category. Here's how to apply for a UPC exemption on these channels:

Amazon - Learn more about Amazon's UPC/GTIN Exemption program and if you haven't already applied, then complete the GTIN Exemption Request Form here.

Walmart - open a ticket with Walmart requesting a GTIN exemption for your custom products and category.  Typically, these requests are only granted for Jewelry sellers.