EDIT: Probably down voted because of length, but I'm pointing out that this is an opinion lots of people have, and there's a reason it exists as a truism. If you pass a law against apps like this, how could you possibly enforce it? The seller's car is already in the spot, and police can't get a real time notification that he's using the app. By the time they realize what happened (if ever) the spot has been sold.

This could only be a problem if people start parking cars they would never use in the city, just to profit from these services.