If, for whatever reason, it's decided to let people take the risk of advertising, you should require some sort of acknowledgement from them that they are aware of the URA guidelines, understand those risks and bear sole responsibility.
By the way, there's a legal battle with Airbnb going on in New York.
The minimum stay there is 30 days.
http://www.nytimes.com/2014/05/14/techn ... .html?_r=0
While AirBnB may have a reprieve for now, once the city tightens up the scope of their subpoena, Airbnb will have to reveal who has been hosting stays for less than 30 days.
If it can happen in NY . . . .