If you're willing to jump through a few hoops, it is possible to purchase a car in Canada and bring it over the border to the United States, then register it to drive legally. Rules on emissions and duty are subject to change, so get updated information from the public agencies you'll be working with in the U.S. You may be able to save money, but costs of modifying your car so it passes muster with U.S. authorities may add significantly to the cost. If the numbers work out, however, you can get a great deal on a vehicle by traveling north of the border.
Make sure that the car you buy in Canada can pass all of the U.S. environmental and safety standards. The car's manufacturer will be able to give you this information. If you need to make some minor modifications for the car to comply, you can still bring it over the border. However, keep in mind that cars built for the Canadian market are not made with an eye toward U.S. regulations, so be very thorough about this step or you may find that you can't register your new car.
Check your state's environmental and emissions standards for cars. Each state has different rules, and not every car can pass an inspection in every state. You can get a copy of your state's rules from the agency that regulates vehicles. California, for example, has very strict emissions rules and your Canadian car may not pass muster if you hope to register or sell it in that state.
Locate a car you like, and start crunching numbers. Convert the cost to U.S. dollars, and collect estimates for converting the speedometer and odometer from kilometers to miles, if necessary. If emissions equipment and other modifications are needed, you may be looking at a large investment.
Purchase your car in Canada, either remotely or by traveling there yourself. You are subject to local sales taxes at the time of purchase.
Prepare to drive your car back across the border into the United States. Collect all documents related to the vehicle, proof of your insurance, bill of sale, and registration in Canada.
Expect to pay a 2.5 percent duty on the purchase value of your car when you get to customs. You may be able to avoid much of the duty charge with the $400 exemption that you are entitled to as a returning U.S. resident.