The situation may be a bit more complicated than first appears. For a start, Marriott can show room availability even when that particular room is not available, and that was before the current issue some properties are facing of Marriott adding phantom rooms to the inventory. On top of that, even if a room were for sale perhaps it was already blocked for a different elite member and the agent didn’t want to have to go through all the explanation with you.

Or they’re just lying to you. Who knows. I would recommend against making accusations of lying until you have evidence though.