MacOS – Reinstall Mac App Store on Mac OS 10.7

Awesome, I figured it out. Run this command, which rebuilds LaunchServices:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user

And everything should be back to good! Immediately after running, the App Store allowed me to re-purchase the app.

Source: MacLife Link

In essence, unmount the volumes with those apps and the App Store will let you download things.

I noticed after the upgrade to lion when my TM volume was connected some apps in the app store appeared to be already installed. So try to disconnect or shut off your external HDD and restart the app store so you can reinstall your apps. After that you can re-activate your TM volume. This isn't normal, since spotlight shouldn't look inside the backup volume and see an app installed, but it happened once so it could happen to you as well.