Seller has the active listings but the Inventory API does not return the details for the items.

Summary

A seller is having the active listings and using the Inventory API to get the details for the listings but the seller is not getting the details for the items with Inventory API calls. 

Detailed Description

The Inventory API is ONLY compatible with the Inventory API. Meaning if an item was listed using any of the tools available via the UI on eBay.com or was listed using the Trading API, they will not surface using the Inventory API.

Seller can use the bulkMigrateListing call to convert existing eBay Listings to the corresponding Inventory API objects. If an eBay listing is successfully migrated to the Inventory API model, new Inventory Location, Inventory Item, and Offer objects are created then you can use the Inventory API to fetch/manage the listing.

For more information, refer to the guide here: https://developer.ebay.com/api-docs/sell/static/inventory/migrating-listings.html