He did sell one of his houses. He only has 2 mansions now.
But rather than doing the socialist thing and giving that extra house away, he did the capitalist thing and sold it. I wonder if Sanders even realizes just how much of a capitalist he is. Especially when he refused to ever work in the private sector because he hated capitalism. He freeloaded and/or lived on welfare until he found his way into public service.
Basically put, even when he was freeloading he still benefited from capitalism. You just cannot escape it. Even in a total communist nation there will always be a black market selling goods behind the government's back.