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.
LOI is fraught with many legal booby traps & pitfalls especially for the buyer. But probably the most damning reason why credible crude buyers would have little or no use for LOI in their buying dealings, is that using the LOI is fraught with many incalculable legal traps and pitfalls much of which could atimes be very costly for, and to the detriment of, the buyer, according to legal authorities and contract law experts.