Currency is a special case of this because most other things have some sort of inherent-use value independent of what someone will pay you for it. In other word

Currency is a special case of this because most other things have some sort of inherent-use value independent of what someone will pay you for it. In other words, for most things, even if the most someone will pay you for it is $0, it may still be useful to you for some reason and therefore not worth giving it away for free.