LOL...I actually worked for a library about 20 years ago who had "library detectives" on staff, who actually would spend their days going from house to house of people who had long-overdue materials to see if they could collect them (and in turn waiving a certain amount of overdue fine).

Another library I worked for did actually send patron's bills to a collection agency if they had over $100 worth of long-overdue materials and hadn't responded to the library's requests for payment. It was only for lost materials though, never for overdue fines.