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If the SKU is found, M2E Pro will try to link your Magento product with the relevant Amazon Item. To do it, the following conditions should be met:
The types of Magento Product and Amazon Item must coincide:
Magento Simple Product or Magento Variational Product with Selected Variation can be linked only with Amazon Simple Product or Amazon Child Product.
Magento Parent Product can be linked only with Amazon Parent Product.
Magento Child Product can be linked only with Amazon Child Product.
2. ASIN/ISBN of the item in your Amazon Inventory must coincide with the ASIN/ISBN of Magento Product (if it is assigned).
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The List action is being executed for Parent Product only in case you sell Magento Variational Product as a new Amazon Variational Product. In this case, a new item (Parent Product) is added to your Amazon Inventory.
If you sell Magento Variational Product under existing ASIN/ISBN, you should not list a Parent Product since it is not added to your Amazon Inventory. Only Child Products will be listed.