To begin with, it's probably illegal to offer kids for sale.... even for a joke.
And consider the resources expended by LEO to track this kid down -- they didn't know it was a joke, and it could have been real. I'm sure there are laws regarding that sort of thing, too.
But how much LEO does it tack to track down?
If it is real offer the money and go meet to pick up the kid. what, about 10 minutes work?