While technically the sale is valid wherever the buyer and the chometz may be, there are some rabbanim (such as Rav Mordechai Shuchatowitz of Baltimore) who try very much to make sure that they are in the same region. This is because the whole sale is almost a virtual sale or legal fiction. So, in order to make the sale more tangible, these rabbanim will make sure that the goy actually could, if he wanted to, get to the chometz that he bought.
(Rav Shuchatowitz also puts on the contract where a key to the house with the chometz could be obtained. He has the appointment of agency, the contract that lets him sell the chometz, written in plain English, and specifies on it exactly who he will sell the chometz to and where he lives. You actually feel like your selling something.)
All this is to drive home to the seller that there is a serious transaction going on here. Most rabbanim don't insist on this, and the sale works without any of this of the seller actually wants to sell his chometz.
(When would he not? If he doesn't keep mitzvos, but he wants to have a kasherus certificate on his store, restaurant, factory, etc.)