Buyer accepts offer and signs it, where offer acceptance is binding depends of the legal jurisdiction. For example English and common law differs slightly from American uniform commercial law on matters of offer acceptance and when performance becomes binding on an offer. The seller can also reply with a pro forma invoice. In fact a pro forma invoice can serve as quote and offer, becoming a more or less binding purchase order upon buyer acceptance and signing. Pro formas are used more in smaller FCL trades, and not in larger bulk trades.
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."