The same scheme has been run in Manchester (England) at the Victoria Station car park for as long as I can remember. Luckily my Italian accent and angry face is enough to keep thugs at bay, but I bet they're tolerated by local police as long as they don't threaten the locals too much.

In order to fight the scam, ticket machines now print your plate number on the ticket itself, so if you buy one from the thugs, you are risking a fine. The fact people keep doing it shows very clearly how parking charges in central Manchester are unrealistically high.