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.
Why? Because my intent can change. I intend to buy the Empire state building, but who cares what my Intent is? Many traders and Brokers treat a Letter of Intent as if it were holy writ. This is a critical beginner's mistake. As its name indicates _ this document expresses an intent.