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.
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.