Summary
The best way to find out if your item can be tracked by SKU is to call GetItem. GetItem returns the field InventoryTrackingMethod only for items which can be tracked by SKU.
If InventoryTrackingMethod returned as "SKU" by GetItem:
The item can be tracked by SKU
And the item can be tracked by eBay ItemID
Additional Resources
Documentation: GetItem Call Reference for InventoryTrackingMethod
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