In selling or buying a used car, private individuals do not have the resources of a trading company with regards to bills of sale, so they have to find forms to download from the internet. In the United States, there are forms downloadable by state in the DMV website.
Generic blank bill of sale forms where the particulars can be filled up by either the buyer or the seller can also be bought from office supply stores. At least two copies should be filled up and signed, one copy for the buyer and one copy for the seller.