Anyone who's ever driven in San Francisco knows that parking in the city can be a nightmare -- garages are often expensive and street spaces are almost impossible to find, especially during peak hours. Several parking apps have cropped up to resolve that issue, a few by allowing folks to auction or sell their spots to others. Sounds like a neat way to make money, right? Well, there's a very important snag: buying and selling public parking spaces like that is illegal. San Francisco City Attorney Dennis Herrera has just issued a cease-and-desist demand today to Monkey Parking, an iOS app that lets users bid for other people's parking spaces. The letter cites San Francisco Police Code section 63(c) that states: "It shall be unlawful for any person, firm or corporation to enter into a lease, rental agreement or contract of any kind, written or oral, with or without compensation, for the use of any street or sidewalk."