The rules have recently changed when importing from the US to Canada.
The US seller needs file an EEI (electronic export information) using the AES (automated export system) at least 72 hours in advance of crosssing the border. After filing, the seller will receive an ITN # (internal transaction number) which needs to be provided to the US border agent along with the title. To complete the AES filing, the seller needs a EIN # (federal tax number). Most private sellers won't have this and need to apply for one with the IRS. Not all private sellers will want to do this so if the seller doesn't have an EIN or won't get one then you can't buy the car.
Shopping for a U.S. vehicle? Beware - Winnipeg Free Press
https://www.riv.ca/USCustoms.aspx
NEW!
As of April 5,2014 U.S. Customs and Border Protection (CBP) has begun a 180 days period of "informed compliance" as all exports of used self-propelled vehicles will be required to submit an Electronic Export Information (EEI) through the Automated Export System (AES).