LOI's are the legal equivalent of "almost pregnant." Letters of Intent emphatically state that. They state that they are not formal agreements, and then often proceed to set forth agreed terms of the proposed transaction. Given this paradox, if the deal goes sour, one party can argue [in court] that those agreed_upon points were, in fact, agreed upon _ or, in fact, a binding contract. And, in some cases, furthermore, that the party relied on the LOI and has monetary damages based on such reliance."
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.