HOW VALID, OR OTHERWISE, ARE THESE USUAL RATIONALE BY SELLERS OR THEIR AGENTS? Ironically, while oil sellers and their agents frequently demand that prospective "serious" buyers involved in crude oil transactions should first offer an LOI, the buyers, on the other hand, are not generally enamored of that idea.
If what you want is a quotation or offer, then sending a letter of intent makes you look ignorant while requesting a quotation will generally at least make you look like you know what it is that you want. And what you want, after all, is a price quote. Hence you want to request a price quote, via a request for quotation.