"Letters of intent are often touted as a 'non_legally binding' way to get the parties to set forth in writing what the undertaking is among them relative to a transaction. Too often, parties will sign such a document, feeling that they have little or nothing to lose by doing so... [True, that's] one of the attractive elements of the letter of intent [its purported non_binding nature]. However, courts have found letters of intent to create binding obligations, even if the letter itself does not explicitly state that it is binding... certain provisions within the document may indeed [still] have legal effect."
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.