Can I sell the same item as new and used at the same time?
If you have the same product both in new and used condition, you will need two different SKUs on Amazon - which is only possible by having two separate products with different SKUs in WooCommerce.
Duplicate your WooCommerce product and set the item condition on the edit product page accordingly - and assign a different SKU for the used item than you use for the new item. Then list both items on Amazon.
See Can I link one WooCommerce product to multiple Amazon listings? for more details.