Yosemite Updates From App Store Not Working I recently bought a new iMac and installed Yosemite on it, but for some reason whenever I try to update I just get this message:

"[Name of App] was purchased using the App Store on another computer. To use [Name of App] on this computer, sign in using the Apple ID and password used to purchase it."

Nope. My Apple ID remains the same as it was on my old iMac, and I don't remember ever having another one. If anybody knows how to sort this out, I'd be very grateful as I haven't been able to update anything in about three months.
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.