The rules have changed recently that make it very difficult. Anything you are intending to take to the US and leave there is considered a commercial import. To process that, you first need to apply for a commercial import user ID for the US Customs system. There's a pretty steep annual fee for this. Once you have that, you can create your online import "paperwork". There's a fee each time you cross the border, plus whatever duty or tax they think you need to pay.
For small things, stick them in the mail.
The only way I would do a deal like you suggest is for the guy to come into Canada, then he brings it into the USA. He can do that without any of the bureaucracy at the border because it is considered to be a personal import (up to $800USD).