The catch there is that it was a private sale. Customs will not charge duty on items that fall under NAFTA (it has to be properly disclosed). However in your case I imagine they were not the original manufacturer boxes nor said made in USA on them. The country where the product was manufactured and (or final assembled/made) has to have those markings from the manufacturer.

You put an item in a generic box and label it so and so. Ship it UPS ground. You will get charged duty.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne