Brain Teaser

Perhaps it's because my brain has already melted, but it took me forever to figure out how this trick works! There has to be some kind of catch! Can you figure it out?

I place $20 in a box.

So do you.

Now the box contains $40, and we both know it.

I sell the box to you for $30. 

And we both walk away with a $10 profit.

If you can't figure it out, maybe I should start doing this to earn some extra cash (hint*hint*)!