For some reason, Amazon and eBay orders are created without e-mail. Amazon and eBay will provide a secure temporary e-mail address to filter spam for customers. This e-mail should be part of the Magento order and is not. Then for example creating Invoice or Shipping will fail with this error.
Fatal error: Uncaught TypeError: Argument 1 passed to Magento\Framework\Mail\AddressConverter::convert() must be of the type string, null given, called in /mnt/storage/webroot/magento/vendor/magento/framework/Mail/Template/TransportBuilder.php on line 428 and defined in /mnt/storage/webroot/magento/vendor/magento/framework/Mail/AddressConverter.php:41 Stack trace: #0 /mnt/storage/webroot/magento/vendor/magento/framework/Mail/Template/TransportBuilder.php(428): Magento\Framework\Mail\AddressConverter->convert(NULL, 'AQUAPRODUCTION ...') #1 /mnt/storage/webroot/magento/vendor/magento/framework/Mail/Template/TransportBuilder.php(202): Magento\Framework\Mail\Template\TransportBuilder->addAddressByType('to', NULL, 'AQUAPRODUCTION ...') #2 /mnt/storage/webroot/magento/vendor/magento/module-sales/Model/Order/Email/SenderBuilder.php(63): Magento\Framework\Mail\Template\TransportBuilder->addTo(NULL, 'AQUAPRODUCTION ...') #3 /mnt/storage/webroot/magento/vendor/magento/module-sales/Model/Order/Email/Sender.php(86): Magento\Sales\Mod in /mnt/storage/webroot/magento/vendor/magento/framework/Mail/AddressConverter.php on line 41