On Offer acceptance there is either a contract generated or a pro forma invoice generated. Since the offer is binding once accepted, the offer if well enough detailed and advised can and often does take the place of the contract. Customarily in Real Estate transactions the purchase offer itself is generally also the contract for purchase as well. The offer becomes a binding contract once accepted.
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. Especially when, in effect, what is being asked of them is to provide the LOI upfront to a little_known Internet_generated seller about whom they lack any familiarity with or whose bona fides as sellers they know next to nothing about _ other than, perhaps, that they (the buyers) had had some initial communication with the "seller" via an Internet contact. In deed, to this writer's knowledge, crude buyers, particularly the more established and prominent ones, would very rarely offer an LOI upfront to any sellers to initiate a purchase. And when, especially, the supposed "seller" that's involved is one that is a virtual unknown to the buyer, or one that is merely an Internet_generated seller about whose bona fides and credentials the buyer knows practically next to nothing, one can be almost absolutely certain that the chances of a crude buyer of substance signing over an LOI to such a seller, is practically next to zero.