Hmmm...it's an interesting idea, but the logistics are...complicated.
There's public property and private property (independently owned stores/parking lots) where this could conceivably be used. On public property...man, trying to work something out with the govt. on ticketing...I just don't see that working. And if you're restricted to private property, there's simply less of a market.
Some cities have reporting sites for a number of things like potholes, roadkill, and in some cases illegally parked cars if it's been a while. People report things to bring them to the attention of authorities/improve the community, but there's no kickback.
If you really wanted to pursue this, you could do some trial runs at maybe a large housing complex or something nearby? Or maybe in suburban places as a way to make HOAs even more annoying ;)
You might think about other ways to monetize it, in order to make this work in most places.
Any small business owners or the like know how money is usually exchanged when cars are towed?
I'm confused as to where the money would come from. The city (or all the cities) would be willing to negotiate a deal with you, the app creator, that they'd give you a cut of the ticket fee after it was paid? Why would they not just take the tip and keep the ticket fee? What happens when the person doesn't pay their ticket? Another app for tracking down people with warrants out for unpaid parking tickets?
I think you could just charge the municipality and the towing company a flat fee to have access to the "tips" from users of the app. Then, you use that access fee to pay back people who tip and take the rest.
That sounds more realistic