When Francisco Nieto and Mirella Rangel sold their house near Mills College this year, they wound up in a limbo of homelessness that's snagged many move-up buyers in the Bay Area's overheated real estate market. For six months, from February to August, they lived like nomads, staying with friends and then hop-scotching from one Airbnb rental to the next while managing the sale of their two-bedroom home and struggling to find a larger one they liked and could afford.