I'm always wary - wouldn't the seller be better off at least opening them to give a better idea of what's in the box? Even an unbuilt model X or Y bike will sell for a lot more than a mystery box.
Of course, if there are bikes (or parts) in there, you can't really lose money at $10 a box. I've no idea about Japan but the price of bikes is really high at the moment due to covid and the impact on supply/demand.