Trade me was losing relevance for younger people in the face of Facebook marketplace. A key problem is that we did'nt let people under the age of 18 join the site (they still join, but either use their parent's accounts to do so or they lie about their dat of birth).
We had 6 hours to come up with a solution as a self-elected R&D project.
We looked at our sign up page, made some suggestions and validated them with business analyst, developers, legal and policing, we all agreed that this was the cleanest approach.