Yosemite Updates From App Store Not Working
You may have an application which was left over from the previous owner.  The simple way to deal with that is to delete the old version and download the new version (with your own Apple ID) anew.

First, in the Mac App Store, go to the Purchases tab and verify that the app you want to download is listed there for your Apple ID.

Next, move (or, if you are brave, remove) the application from the /Applications/ folder to someplace like your Desktop.

Finally, quit/restart the Mac App Store, go back to the Purchases tab, and re-download the application.