Ebay: Inventory not syncing due to migration of listings to eBay Inventory API

Error message from eBay:

"This operation is not allowed for inventory items. Inventory-based listing management is not currently supported by this tool. Please refer to the tool used to create this listing"

Error code: 21919474

Here is a Full list of errors messages given by eBay.

When does this error occur?

If a listings migrates to Inventory API, it cannot be updated using the original eBay API that Sumtracker is integrated with. When your listings have been created or updated using a different tool that uses eBay's Inventory API, it automatically migrates the listing to Inventory API. (Sumtracker uses the 'Trading' APIs.  APIs are what we use to communicate messages with eBay).

What's the problem?

eBay unfortunately didn't make the Inventory APIs compatible with the trading APIs.  This means that no tool that uses the trading APIs including using eBay directly can update listings created with the Inventory APIs.

Why doesn't Sumtracker use the Inventory APIs?

Because the Inventory APIs are not feature complete - you can't do all the operations that merchants need to manage listings using the Inventory APIs.  If we moved to the Inventory APIs there would be many complaints from merchants about inventory updates using Sumtracker.

Why did eBay create a tool using Inventory APIs that aren't feature complete or make them backward compatible with the trading APIs?

Great question and one for eBay management.  If they had not done either of those things there would be no problem but as it is Sumtracker is stuck with not being able to update inventory of listings migrated to the Inventory APIs.  

So what's the solution?
There is no way to update the existing listings using Sumtracker (or anything except the original tool that used the Inventory APIs).  The only solution is therefore to end the original listings created with the Inventory APIs and create new listings directly on eBay for them to work with Sumtracker.  Ending existing listings can either be done in the old tool (assuming it's still installed) or directly on eBay.