The point is that the often_heard notion and claims by some sellers or their overzealous agents and brokers that the use of the LOI to initiate a purchasing proposal by a buyer "is how it is usually done and this is how it should be," may be applicable and prudent only in the minds, the imagination, and hopes or dreams of those sellers, especially the more marginal ones and their brokers and agents who operate on the fringes largely on the Internet. It is NOT a view that is shared by the broad spectrum of credible buyers, more especially when the "sellers" involved are largely unknown and obscure operators.
So let's sum things up: The LOI and similar documentary terms, are non standard, confusing, and lead to ambiguity. Documentary ambiguity is referred to in trade law as a condition "The Battle of the Forms" in which confusion from contradictory and ambiguous forms and documents damages the transaction. This should be avoided. To seasoned corporate traders and bankers, using terms like "Letter of Intent" is the mark of amateurs. What you properly want is an RFQ, request for quotation, from a seller or supplier. The seller may then reply to your RFQ with a quote OR full offer if they wish.