Right, but for the average user, it's not really feasible for them to receive a megabytes-large CSV of data in whatever format Amazon decides to provide, and then transform that into the necessary format for import into <Etsy/Ebay/Shopify>. We could make tools to do so, but then Amazon could just change what format they use for export, so we'd need to update the tools...

Regarding importing: you as a user have the right to provide your downloaded data to a different party, but there's nothing in GDPR that says that party must offer the necessary import capability.