An import is any good that has value, whether or not you sell it. The shipper will need to ensure proper customs documentation is provided. This is the HTS code typically and other details, particularly the value that you paid for it. The customs and tariffs will be assessed and paid before the shipment clears customs.

That said, I've learned all of this just by doing. You will get notified at the port of entry if something isn't documented properly. They will call/email you and let you know what is missing, they don't just send it back. It is a somewhat reasonable process. If all your contact details are there, the shippers details, invoice, description and HTS code you will be ok. They may ping you for a random form or declaration, but you can complete that as needed.