App store problem after updating OS to Mavericks. Ended up deleting app store. How do I get it back?

Question

I upgraded to Mavericks.  When I tried to update my apps in the appstore, I was getting prompted to enter in a password for someone else's Apple Id.  I have no idea who this person is and I could not change the ID.  I was logged into the App Store under MY id but for whatever reason when i chose to update an app, I was prompted with this person's ID. I researched this and someone in a forum suggested deleting the app and reinstalling it.  I have app zapper so I zapped the app and I no longer have an app store..  I also have no way of getting the app store back.  Apparently, it's part of Mavericks.  I cannot even reinstall Mavericks because Mavericks is only attainable through the App store.
I have a big problem don't I?
I recovered the files that app zapper "zapped" from my trash.  They are as follows:
One "app store" folder that contains a file called: updatejournal.plist
One "AppStore" folder that contains a file called: manifest.plist
And a file called com.apple.appstore.plist
How can I get app store working again?
And then there's the issue with this person's apple ID.  How do I get the app store to purge this person's login?  My machine is an Apple Refurbished so perhaps the person who is attached to the store is the person who bought the machine new?  I have no clue.  I do think it's a huge security flaw on Apple's part to expose a stranger's login information
Help is immensely appreciated.
Thanks.

Answer 1

There are a few things you need to do.
A.
Remove "App Zapper" according to the developer's instructions. Never install it, or anything like it, again.
B.
Deleting system components is never the solution to any problem.
If you don't already have a current backup, back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.
If you use FileVault 2, then before running the Installer you must launch Disk Utility and select the icon of the FileVault boot volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another icon with the same name. Click the Unlock button in the toolbar and enter your login password when prompted. Then quit Disk Utility to be returned to the main screen.
If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode.
*The linked support article refers to OS X 10.9 ("Mavericks"), but the procedure is the same for OS X 10.7 ("Lion") and later.
C.
You installed a hacked app, originally from the Mac App Store. It contains the receipt for a different app, downloaded using an account that you don't control. You need to identify and remove the hacked app.
Important: The app you need to remove is not necessarily the one named in the App Store alert. For example, the App Store may prompt you to update "Angry Birds" or "Twitter," but the hacked app may be something else entirely. Don't make any assumptions about which app you're looking for. To find it, you must carry out a systematic search with Spotlight.
1. Triple-click anywhere in the line of text below on this page to select it:
kMDItemAppStoreHasReceipt=1
Copy the selected text to the Clipboard by pressing the key combination command-C.
2. In the Finder, press command-F to open a search window, or select
File ▹ Find
from the menu bar. In the search window, select
Search: This Mac
from the row of tokens below the toolbar. Below that is a popup menu of search criteria, initially showing Kind. From that menu, select
Other...
A sheet will drop down. In that sheet, select
Raw Query
as the criterion, then click OK or press return.
Now there will be a text box to the right of the menu of search criteria. That's where you enter the raw search query. Click in that box and paste the text you copied earlier by pressing command-V.
4. The search window will now show all the App Store products that are installed. Compare those search results with the list of your purchases from the App Store. To see the complete list, you may need to unhide hidden purchases. If any apps were download from the App Store using other Apple ID accounts that you control, sign in to the store under each of those ID's and check the purchases.
At least one of the apps in the Spotlight search results is not among your purchases in the App Store. Move each such item to the Trash, after quitting it if it's running. You may be prompted for your administrator password. Empty the Trash.
Quit and relaunch the App Store. Test.
If you find these instructions confusing, ask for an alternative method.