Hey Superdave! When I first got out of law school, I worked at a law firm where the owners also owned a collection agency. They would buy the bad debts and collect on them. There is lots to say about it, and, yes it was a profitable company. There is lots to know about the law in that area, and one of the reasons that the company was so successful was that the law firm would then sue on the bad debt.
Once paper gets so past due that it is sold to a third party, the other company has usually called, mailed letters, threatened suit, etc. We most often got the money from suing, getting a judgment, placing liens, garnishing wages, issuing property executions, etc.
If you have a lawyer or group of lawyers to use, it can be a profitable business. You have to know what you are doing when you examine the collectibility of a portfolio for purchase. Sometimes the docs are defective or whatever, and you can waste lots of money that way.
HTH