Seller replies with an offer (the firm or full offer is the so_called "FCO" in Internet Broker_speak).
But intentions are not binding contractual conditions. Nor need they lead up to binding contractual conditions. Moreover a letter expressing intent, in its form, does not adequately suggest what the appropriate response should be. Whereas a Request for Quotation / RFQ quite literally is a Request. For a Price Quotation. Hence the document's name, it meaning is expressed by it's name. Hence an appropriate response, for a vendor or supplier, is issuing a a price quote.