getInventoryItems is for fetching items listed via the "new sell api" have you listed anything via the "new sell api" or bulk migrated trading API listings. Feels like you only have "trading api" listings which cannot be recalled via the "new sell api"
@fredaldous, thank you for your response. The company where I work have currently about 100 000 ads on Ebay (FR). I must export all fields of all ads. If I understand your response, it's impossible. Have you a track for doing that ? Thanks