What should be done about the mezuzot (internal and external) when selling one's house...
- to a non-Jew?
- to a non-observant Jew?
- to an observant Jew?
To a non-Jew: take them down.
To a Jew: you're supposed to leave them up, and you can charge him for the cost. (If the buyer refuses to pay for them, you can take them down.) There are differing opinions whether, if you have super-special mezuzas up there, you can replace them with ordinary mezuzahs; what I've heard is mostly that it's permissible.
I recently read and liked this little essay on the topic, written by Rabbi Joan Glazer Farber and published by the Union for Reform Judaism.