A bill of sale isn't the same as a sales contract. A sales contract is drawn up and signed before a transaction. It lays out the terms of the transaction before buyer and seller come to a final agreement.
A bill of sale is more akin to a receipt rather than a contract. Since it is such a simple document, there is no way to truly enforce it. If the bill of sale is the only document you use in a transaction, it is best to ensure there is enough detail on it to make the transaction enforceable.
Whatever the case may be, selling a no-title car is illegal in most states, including Texas.