In a letter to Paolo Dobrowolny, the CEO of the Rome, Italy-based tech startup, city attorney Dennis Herrera cited a provision in San Francisco's police code that prohibits people and companies from buying, selling or leasing public on-street parking and mandates fees of up to $300 for drivers who violate the law.

Herrera's warning to Monkey Parking is the latest attempt by city government officials and state lawmakers nationwide trying to figure out how to regulate Web-based businesses that offer shared parking, transportation and housing services using mobile applications. Among the more popular ridesharing services are Uber and Lyft, and popular housing apps include Airbnb.

The Monkey Parking app allows drivers to get an often elusive parking spot and sell it for $5, $10, even $20, and then wait until the buyer arrives to take their place.