1. If you are using Mavericks, uninstall Memory Clean. It will interfere with the unique and beautiful way Mavericks time-compresses memory. All optimizers, cleaners, boosters, keepers, guards, and so on, should be avoided as the plague.
Having a Mac on Appstore does not mean Apple supports it, the only restrictions that Apple has there is that it is programmed correctly to interface with OS, and that the developer has a identification with apple.
2. Similar issue: Sign out of the Appstore, Quit the Appstore (from the menu), logout of your account then login again (or restart), then start Appstore again, login, and try again.
If that does not work, quit the running app, move the app in question to the Trash bin (use no app deleters), empty the trash, then try again.