Concerning the kushya mentioned in April 4th's post:
During a discussion with Nachum I realized that I had not read the halacha carefully. It is significant that the Rambam's case is a renter. When one is renting from a Jew he may not remove the m'zuzot upon leaving; when he is renting from a non-Jew he must. It would seem to follow that if one sold one's house to a fellow Jew it would be permitted to take the m'zuzot.
I have not done extensive research to see if everyone agrees that this halacha only applies in the case of a rental but a quick scan of the Aruch HaShulchan (Siman 291) seems to bear out my contention.
The solution to the kushya mentioned in the previous post must explain this.