Finally, there is no such thing as an oral offer! An oral offer is really just a test by the buyer to see what the seller will or will not accept. Make the buyer put the terms of the offer in writing as an intermediate step toward securing a legitimate offer.
To conclude, there's perhaps no more apt way to conclude this piece, than to quote this very fitting statement by contract law attorney, Ivan Hoffman, of California: "[Given that] the letter of intent is essentially a legally worthless document [but yet one that could potentially cause many serious legal problems for the signer]. It is not clear to me the reason any party would ever bother to create such a document and yet I have seen it used on many occasions."