Don't try this. It can land you in big trouble, or at least your new purchase stuck at the border for a period of time while they sort it all out. I heard a story about something like this that went wrong, and only got worse because of stupidity on the part of the guys trying to get the car from Canada to the US. I don't think anyone went to jail, but they did end up having to shell out cash to get their car home.

I would get all the specifics nailed down from someone official (not just an internet forum) about getting the car over the border before making the deal.