Transporting a newly purchased car from USA to Canada
Guess I should start by saying this is based on you bringing the vehicle into the country yourself, whether towing it or driving and the vehicle being 15 years or older.
A photocopy of the title must be faxed to the US Port of Entry where you are coming back into Canada at least 72 business hours before you plan to bring the vehicle across and you must use that POE. Hopefully the seller would agree to fax that for you unless they're willing to send you the title before hand and you can fax it yourself. Always a good idea to check before hand what the hours of operation are as they may not do vehicle exports past certain times of the day or on weekends. When you get to the border before you cross you take the original title into US Customs office, tell them you exporting a vehicle, they hopefully find your faxed copy of the title which they use to check that it's clear and not stolen or anything, if all is good they stamp it and you're on your way to Canada. They may or may not check the vehicle, I've never had them come out and check any that I've brought over but they might.
On the Canadian side you'll tell them you're importing a vehicle, you'll give them the title and your bill of sale, they'll fill out some paperwork, probably ask if it has A/C, which is an automatic $100 Federal tax, you'll give them some money for that and the GST based on the price you paid in US converted to Canadian and that should be about it. Not sure how other Provinces do it but in Ontario when we get the title transferred we pay provincial tax based on the selling price, your Province may be different.
Really not much to it but there's two big things to remember. The 72 hours is based on business or working days so if the copy of the title was faxed on a Friday morning it would be Wednesday before you could bring it across. They can hold the vehicle until they check the background and/or fine you. Last I heard I think it was $500US.