The same Merchant SKU was found among 3rd Party Listings. Merchant SKU must be unique for each Amazon item

If you get the error: "The same Merchant SKU was found among 3rd Party Listings. Merchant SKU must be unique for each Amazon item.", it means you have such item already presented on Amazon. So M2E Pro prevents it to be listed to avoid problems.

You should perform next steps to solve this problem:

  1. Apply action Stop on Channel / Remove from Listing, to remove such item from  M2E Listing
  2. Go to Amazon > Listings > 3rd Party Listings and move necessary mapped items to the appropriate M2E Listings

Then already listed item will be moved to M2E Listing and can be synchronized.

Note: If you want to list magento product, which is already on Amazon (same SKU), but use another ASIN, then you should  set option Generate SKU in Listing Settings to Yes.