Forgive my ignorance but if a buyer indicates interest, is it ok for the seller to ask for a LOI? Or if the buyer doesn't mention it explicitly, does it mean t

Forgive my ignorance but if a buyer indicates interest, is it ok for the seller to ask for a LOI? Or if the buyer doesn't mention it explicitly, does it mean that they're not serious enough? Who should mention the first amount of money first, buyer or seller?