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The ASIN/ISBN is already assigned to Magento Product with a help of the Search Tool.
The ASIN/ISBN and (or) UPC/EAN is specified in the Listing Settings > Search Settings > Identifiers Settings Product Identifiers settings.
If the data is valid, M2E Pro will search for the Product on Amazon and create a corresponded corresponding item in your Amazon Inventory.
If ASIN/ISBN is neither assigned nor specified in the Search SettingsProduct Identifiers settings, M2E Pro will check whether Magento Product is ready for the creation of a new Amazon Product. New Amazon Product is created only in case certain requirements are met.
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