If you have a room or second house to spare, you're sitting on a potential gold mine. So just post a rental notice and watch the money flow in, right? It's not quite that easy, but many people earn extra income by listing their spaces on peer-to-peer online rental marketplaces such as Airbnb and HomeAway. Some even stay at friends' homes several nights a month while their own homes are rented out. Here's what potential hosts need to know about the time and costs involved, as well as the ups and downs of the experience.