iPhone In-App Purchase Store Kit error -1003 "Cannot connect to iTunes Store"

I had a similar situation and dumped the iPhone's network traffic to see what's going on. I found that the normal store was contacted instead of the sandbox. It helped to delete the app from the device, make clean and build/install it again. Apparently something with the development profile had gone wrong.

Update: To dump the network traffic of a non-jailbroken iPhone, just use Internet Sharing on your Mac and configure your iPhone to use your Mac's WiFi. Then tcpdump -n -i en1on your Mac will do the trick.

If you're getting error 0 "Cannot connect to iTunes Store" (which I realize is not the error -1003 that OP asked about, but a web search for error 0 also leads here), this can be caused by two possible problems:

1. You're passing in the wrong product identifier. If that's the case, then you'll get error 0 shortly after calling -[SKPaymentQueue addPayment:], before you get the popup asking you to confirm payment.
2. Your test user has become invalidated. This can happen if you accidentally log into the App Store with your test user. When this happens, you'll get error 0 after entering your password to confirm your payment.

To fix problem #1, pass in the correct product ID. To fix problem #2, create a new test user on iTunes Connect, and optionally delete the old test user.

I had a similar issue, was getting "Cannot connect to iTunes Store" with Code=0. This is how I solved it, after trying every other ritualistic piece of advice found on the Internet from re-downloading my certificates to resetting all my device settings to sacrificing a virgin black goat:

In the target summary for the app, I used to have my version number in the "build" field, but nothing in "version" field. This seemed odd to me, so I moved the version number from "build" to "version". This resulted in the aforementioned error. Moving the version number back to "build" solved the error.