I don't know about a car but I had no problems buying and bringing back an ATV from Canada.
Had to pay the GST but received a refund at the border. I registered the ATV in my state with no problems.
My Ford pickup was made in Canada and purchased from a US dealer.
Come to think of it... I had a Sportster once that was originally sold in Canada and bought used by the person I bought it from who brought it back to the US.