You need a car bill of sale in Ontario to prove that you legally own a car. Why Do You Need a Bill of Sale in Ontario? Here’s everything you need to know about why and when you need a bill of sale. The process of creating a bill of sale is surprisingly simple. However, a lot of confusion can stem from the fact that there’s no standard format for a car bill of sale in Ontario. This form is essential to proving that you have legally bought or sold an automobile. A car bill of sale is a legally required document you need to have ready whether you’re buying or selling a car in Ontario.