Presumably they end up on Judge Judy. More seriously, sounds like something that could be handled in small claims court if there's really a dispute about it.
It can't be handled in small claims court because the claims aren't small and exceed the limits of what a small claims court can rule on - I mean, if someone has spent 20 years in that 'common law marriage' then that generally comes to the full extent of divorce proceedings - how to divide up the house, car(s), other property and custody of children.
I should have been more specific. I was referring to the question about what happens when roommates buy a couch together in order to illustrate it's not quite the same.