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 a time, especially in a case involving a supposed seller who is either a fake seller or does not actually have the supposed crude in hand yet, or, an unscrupulous aspiring seller's agent or broker who actually has not acquired a crude supplier (seller) yet, buyers may issue an LOI only to find out that there is no seller on the other end. This happens a lot in situations where you have an hungry agent or facilitator who is still struggling to get a real supplier, and by extracting this LOI from an unsuspecting buyer, this facilitator can commit the buyer only for him then to start hustling to find a seller or supplier.