If you decide to uninstall an app purchased from the Mac App Store (MAS), you may confront some unexpected hassles--especially if you are still running Snow Leopard.
My difficulties with uninstalling began as the result of an entirely separate matter: my MAS purchases were made from two different accounts. Most were made with my own account, but a few were made using my wife's account. This meant that if an update came out for an app purchased from my wife's account, while I was logged into my account, I had to switch logins to get the update. And vice versa. This has led to an irritatingly frequent need for account switching.
I finally decided I would solve the problem by uninstalling the apps purchased from my wife's account and re-downloading them from my own. As the apps involved were either free or of minimal cost, I wasn't concerned about possibly having to pay twice for the same app.
My first step was to get rid of the apps from my wife's account. To do this on my Mac Pro, which is still running Snow Leopard, I simply dragged each app's icon from the Applications folder to the Trash and emptied the trash (entering my admin password, if requested). I next confirmed that any duplicate copies of these apps were gone from the drive.
Back in the Mac App Store, the results of this manual uninstall were inconsistent. In some cases, it appeared to work as intended: the app was no longer listed as "installed" in Purchases. However, in other cases, the app unexpectedly continued to show up as "installed." I am still not sure what accounts for this difference.
More critically, for the apps still listed as "installed," nothing I tried could get the status to change. As a result, the MAS would not allow me to repurchase these apps, from either account.
Even for apps that did shift from "installed" to "install," success was not guaranteed. In one case, when I clicked the app's Install button, an error message popped up that (wrongly) stated that the app "was not purchased from the Mac App Store." When I clicked the Buy button, the attempted purchase failed.
Thanks to a Google search, I finally . I deleted Mac OS X's Launch Services cache, by using the following Terminal command:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
I could now reinstall all the deleted apps--from either account. However, note this potential negative side effect of trashing LaunchServices cache: A few document types opened in an incorrect app. I fixed these via the Finder's "Always Open With" command.
If you are running OS X Lion, the situation is considerably simpler. You uninstall apps via Launchpad. This not only removes the app from the Launchpad display but eliminates it from your drive entirely. That's right. The app is not merely moved to the Trash but immediately deleted. After doing this, you should now be able to reinstall an app--even from a different account if you wish. No Terminal commands or other workarounds are required. At least that's how it worked for me.
There are third-party utilities that can assist in making sure that all traces of a deleted app are gone (such as AppZapper and AppDelete). However, I haven't found that these are necessary or even helpful for this particular task.
My bottom line recommendations for minimizing trouble with MAS purchases: Run Lion, delete apps from Launchpad, and purchase apps from only one account.
An additional mystery: In one case (Minesweeper!), an app I purchased with my wife's account shows up in Purchases even when I am logged into my account.
In another case (Chopper 2), also purchased from my wife's account, the app is not listed when logged in to my account. I only see it if I log in to my wife's account in the Mac App Store app.
Both apps show up in the Updates section, when an update is available, regardless of what account is currently active. And in both cases, I need to switch to the purchasing account before I can actually obtain the update.
Solution: It may be that I purchased Minesweeper from both accounts (I can no longer recall for sure), which is why it shows up as Purchased in both accounts. However, the currently installed copy is assigned to only one account; this is the only account where updating is possible. By uninstalling it from one account and reinstalling it from the other, you can switch this.