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.
Among Internet based brokers and agents the LOI is commonly advanced as a leading inquiry document. This protocol is utterly incorrect, and misuse of the LOI is often seen by experienced Exporters as a mark of an amateur. Letters of Intent are not standard international business documents, though they do legitimately pop up rarely, much less often than most brokers believe.