If you can stumble across the lone worker in the capitol can buy it, you can hurt them. This is something than people use when they can. It is not that the AI wants to sell the worker, but it foolishly sends the worker back and forth and it ends up in the capitol.
It can also happen that barbs chase the worker in, but at DG or better barbs will get waxed. Just check frequently in the very early part of the game. Of course you need 120 gold in C3C.