Some states require only the buyer to sign the document, while others require both the buyer and the seller to sign the bill of sale.  Possession of the signed bill of sale and payment of the title transfer fee may be required to apply for a title at the state's DMV.
Other states call for the form to be notarized. You would need to look at the State requirements. However, it should be noted that without the buyer's signature, the Bill of Sale may not be an enforceable contract.