It's much more widespread now with Airbnb than with Craigslist. Craigslist is more promiscuous and serendipitous whereas Airbnb is more corporate and explicit. If a whole company that is valued at $10b has a business model for this, one would assume that such a service is completely legal. That's the heart of the issue here. It's gotten to the size now where media, landlords, and government have to step in before it becomes a black market of sorts.