You can check a few things yourself to find out if it is worth paying a mechanic to look at the car. The Automobile Protection Association recommends getting a written account of the car's accident history from the dealer and private seller. Watch for paint color consistency and paint bubbles. Also ask for the car's service records. Compare the mileage on the record to the car's odometer.
Odometer fraud is a reason why curbside sellers can let a car go for a seemingly great price.
By law, dealers must guarantee against stolen vehicles and against liens on the car. For private sales in Ontario, the seller should provide you with a Used Vehicle Information Package that shows ownership as well as liens on the car and the financial institution that has that lien. If the UVIP is not provided, go to any licence bureau in the province and get one before buying the car. Otherwise, you may have paid for a stolen car, which you won't get to keep. Only Ontario has a UVIP for private sales but the APA says that each province has a database potential buyers can access, for a fee, to check a vehicle's background.