I was wondering a similar thing about land within our own towns.. I am choosing to apply the golden rule to this:
(for those of you that don't know the golden rule: "unto others as others unto yourself") and would like to know if this would be an acceptable method for reclaiming inactive land within your town..
1. Is the player inactive? When were they last seen, check their profile.. at least 30 days since last seen for me before I start to make assumptions about a person's inactivity in-game. Even if they have been inactive,
2. Msg the player in/out of game: give them plenty of time to respond. Let them know why you would like to reclaim their build: i.e. "Dear so-and-so, I was wondering if you were still using your build in my town, if not I would like to resell it, is that ok."
3. Screen shot the build, and look for changes. Maybe the player has just moved to another build somewhere else. ScreenShot from all sides so that should you remove it and they want it later, you can provide the screenshots to them so they can duplicate it in their new place.
4. Did they pay you for the land? If yes you should be prepared to refund them at least this, if not more if they suddenly return. This also goes for the materials collected should it come time to remove the build.. they should get the mats back if at all possible
5. Don't take it lightly and be patient.. offer them the opportunity to keep it with no conditions: the only reason to remove a build should be due to inactivity, not any personal preferences or design quality etc etc (unless those were conditions of the initial sale: i.e. "no cobble" then they use cobble, it is fair game."
Remember, while you may have build permissions (and destroy permissions) protected chests and inventory within the build is not necessarily fair-game too.. This stuff should be kept for safe-keeping for at least some period of time, not just turned around and used or sold, especially valuable items and rarer items like diamonds and spawners, because in my opinion, even in your own town, this would be stealing.
Do some form of documentation. Whether it be screen-shots or messaging, C-Y-A ... If you put all this effort into treating your town's citizens right it will help justify your reasoning in the case of a dispute and will show future citizens that your aren't an ass..
Sorry if I got long winded, I wasn't intending on hijacking your post..