Thanks for the comments. This site wasn't my idea, I found it on a dutch startuplist site and was curious what others thought of it.
This does not seem like a safe idea.
Indeed. The item might be something like a locked phone that has child porn on it. If customs asks you to unlock the phone but you claim that you you can't, it would make them instantly very suspicious of you. Who would want to take that risk?
I'd only carry an item for someone I knew and trusted.