A handwritten bill of sale can be a legal document as long as the bill of sale has been signed by both the buyer and seller and be sure that the VIN number and license plate number are on the written bill of sale and it's also a good idea to have the bill of sale notarized.
You can either get a bill of sale printed out or yes you can write out a legally binding bill of sale so long as it's done correctly and the handwritten bill of sale will work just as good as a printed out bill of sale.