Handwritten is legal — but that's the wrong question
Courts and DMVs care about what a bill of sale says, not what it was written with. A complete, signed handwritten document on notebook paper is valid in most circumstances. The real risk is what handwritten documents tend to leave out: the VIN, the odometer reading, the as-is clause, the exact legal names.
What it must include, handwritten or not
Full names and addresses of buyer and seller; a description that identifies the exact property (year, make, model, and VIN for a vehicle); the price and payment method; the date; and both signatures. For vehicles, an odometer statement protects the seller.
Why printed wins in practice
Registration clerks process thousands of documents; a clean, printed bill of sale with every field labeled gets accepted without questions. A cramped handwritten note invites scrutiny, callbacks, and repeat DMV trips. Since generating a printed one takes two minutes, there's rarely a reason to hand-write it.