It started in a school WhatsApp group.
My son's school in Barcelona has an intercambio — a WhatsApp chat where parents pass on outgrown clothes and toys. It works. It just barely works. Things get buried. Items go to whoever happens to be online at the right second. There's no record of what's available or who claimed what.
But the underlying idea — parents helping parents, things kept in use, nothing thrown away that's still good — that idea is right.
Outgrown is the version of that group built properly. Bigger than one school, smaller than the whole internet. Local enough to meet at the park, large enough to actually find what your kid needs next.