For data import, I highly recommend using AJW datagrab with CSV data.
data export is a little bit trickier, but you can setup a template to output to CSV/XML or you can use a plugin like Zenbu Studios Hokoku to setup exports directly from the admin CP (if you use Zenbu).
If you want to setup CSV output and file download via a template you can use a plugin like http header to set the content disposition.
Check out the http header plugin here: https://github.com/rsanchez/http_header
While the above plugins and methods work great for nearly all EE data, CT order data works a bit differently - specifically order_items/item_options.
Item options are stored in the db as base64 encoded data and must be serialised or unserialized as needed. via php:
you may be able to serialize your data for item_options prior to export and then import it directly, but I have not tried doing so and cannot vouch for an easy way to handle it. However, if you are simply using the basic item_options tags (e.g., size, S M L XL; color: red,green,blue etc.) you may be able to work directly with that data via datagrab. Item_options becomes much more complicated if you are using many custom options.