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.
"Parties to a transaction sometimes intentionally create a letter of intent as an expression of what they intend to agree upon should certain circumstances arise... [whatever happens], the document will not be binding and thus not enforceable until those circumstances arise. Thus, the letter of intent is essentially a legally worthless document. 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. If parties to a transaction intend to bind each other, then they should create a binding contract, not a letter of intent. If the parties to a transaction do not intend to bind each other, then why bother creating a document that is not binding?