> What happens if another guy with one of these locks comes along and parks his park on the same post as yours?

Presumably, if these were mass-produced, each device would have a rare or unique signal similar to car remotes.

> Also as far as I can tell this lock relies on the poles/posts being of a certain diameter

I thought of that too. There are standard light pole sizes, but they might be different in different countries. I wonder if it would work s well on wooden posts.