getPurchases() NullPointerException initializing mService

Question 1

The reason this was not working was because onServiceConnected() isn't guaranteed to be called until onCreate() completes. So I moved the try/catch code into onServiceConnected(). I'm not sure if this is best practice, but it seems to have fixed my issue. Here is the resulting code for anyone finding themselves with this same error. This is exuding button and purchasing logic. I recommend this tutorial for that code.

public class MainActivity extends Activity
{
    IabHelper mHelper;
    IInAppBillingService mService;
    static final String ITEM_SKU = "android.test.purchased";
    private Button buyButton;
    AdView adView;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        adView = (AdView)this.findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().build();
        adView.loadAd(adRequest);

        buyButton = (Button)findViewById(R.id.buyButton);

        bindService(new Intent("com.android.vending.billing.InAppBillingService.BIND"), mServiceConn, Context.BIND_AUTO_CREATE);      
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu)
    {
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    ServiceConnection mServiceConn = new ServiceConnection() 
    {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service)
        {
            mService = IInAppBillingService.Stub.asInterface(service);
            Log.d("TEST", "mService ready to go!");
            checkOwnedItems();    
        }

        @Override
        public void onServiceDisconnected(ComponentName name)
        {
            mService = null;            
        }
    };

    private void checkownedItems()
    {
        try
        {
            Bundle ownedItems = mService.getPurchases(3, getPackageName(), "inapp", null);

            if(ownedItems.getInt("RESPONSE_CODE") == 0)
            {
                ArrayList<String> ownedSkus = ownedItems.getStringArrayList("INAPP_PURCHASE_ITEM_LIST");
                ArrayList<String> purchaseDataList = ownedItems.getStringArrayList("INAPP_PURCHASE_DATA_LIST");
                ArrayList<String> signatureList = ownedItems.getStringArrayList("INAPP_DATA_SIGNATURE");
                String continuationToken = ownedItems.getString("INAPP_CONTINUATION_TOKEN");

                if(purchaseDataList.size() > 0)
                {
                    //Item(s) owned

                    for(int i=0; i<purchaseDataList.size(); ++i)
                    {
                        String purchaseData = purchaseDataList.get(i);
                        String signature = signatureList.get(i); //Note signatures do not appear to work with android.test.purchased (silly google)
                        String sku = ownedSkus.get(i);
                    }
                }
                else
                {
                    //Item(s) not owned

                    String base64EncodedPublicKey = "public_key";

                    mHelper = new IabHelper(this, base64EncodedPublicKey);

                    mHelper.startSetup(new IabHelper.OnIabSetupFinishedListener()
                    {
                        @Override
                        public void onIabSetupFinished(IabResult result)
                        {
                            if(!result.isSuccess())
                            {
                                Log.d("TEST", "In-app Billing setup failed: " + result);
                            }
                            else
                            {
                                Log.d("TEST", "In-app Billing is set up OK");
                            }
                        }
                    });
                }
            }
            else
            {
                //Error checking owned items
            }
        }
        catch(RemoteException e)
        {
            e.printStackTrace();
        }
    }

    @Override
    public void onDestroy()
    {
        super.onDestroy();

        if(mServiceConn != null)
        {
            unbindService(mServiceConn);
        }
    }
}

Question 2

Your code just says

IInAppBillingService mService;

It is null, you need to initialise it.

Put a System.out.println() in the onServiceConnected method to see if it is being initialised. If it is not, it means that onServiceConnected is not being called so the bindService method might not be working as intended

EDIT: I think i know the problem.

Add this outside the onCreate method as a class variable...

ServiceConnection mServiceConn;

Then change

ServiceConnection mServiceConn = new ServiceConnection()

to

mServiceConn = new ServiceConnection()