The contractor’s signature isn’t on the bid; only mine. Is this a legally binding document?
For the most part and to a limited extent, verbal contracts are just as valid as a written contract. Some states may however, impose limitations on the verbal contract if the value of it exceeds a certain dollar value or the term of the contract is over a certain period of time. In other words, holding someone to an agreement to provide a service ten years down the road or a multi-million dollar verbal agreement would probably be very tough in most states.
The problem with verbal contracts are that they are often very hard to prove, especially if they are very complicated or have no independent witnesses. The parties themselves may not even recall the exact details to which they agreed. Therefore, if neither side can can show a written contract, judges are often forced to apply “fairness” or other governing state laws. The problem with “fairness” is that at least one person won’t consider it "fair.
In general, try to avoid verbal contracts. Anyone that wants you to agree to something but won’t put it into writing, just isn’t worth the time. And they are very likely out to scam you.