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.
Many newcomers to the world of international business are given confusing advice on the proper documents to use in their business. The Letter of intent / LOI is a document that many refer to, but actually is poorly understood. Without proper understanding of the document you might find yourself making mistakes in your business that will cause you great embarrassment.