We have a 10m kid. When friends and family are visiting us, we usually receive a lot of toys. If you have a kid I am sure you know that different children like different toys and other they just put aside. I would really love to create a website and a mobile app where parents can post their unused toys and swap with other parents that don't have such toys. It can work the same way for clothes.
As a monetization method, I can suggest a monthly fee to have access to the listed toys, to be able to enter GEO location, etc...