Specifically this part:

> Consideration cannot be illusory

A contract without consideration is void. If you promise to give someone X (that you own) if they give you Y (that they own), and you never give them X, then you don't own their Y, even if it's already in your possession.

You just (non-forcefully) stole their Y—same as if you e.g. asked to borrow someone's phone and then walked away with it.