The endpoint used to work, but I can't make it work anymore.
I don't have many details, because the returned body is empty, but this is the request:
URL: https://api.numista.com/api/v3/users/200326/collected_items
Body: { "collection": 44572, "type": 1492, "quantity": 2 }
Headers: contains “Numista-API-Key”, “Authorization” and “Content-Type”