Android Signed APK showing as Unsigned APK when trying to upload to production

Nothing. Read the documentation: Publishing Updates on Android Market

Before uploading the updated application, be sure that you have incremented the android:versionCode and android:versionName attributes in the element of the manifest file. Also, the package name must be the same and the .apk must be signed with the same private key. If the package name and signing certificate do not match those of the existing version, Market will consider it a new application and will not offer it to users as an update.

This error may be caused by several reasons.

Here is the list of requirements for the Google IAB testing.

Prerequisites:

1. AndroidManifest must include "com.android.vending.BILLING" permission.
2. APK is built in release mode.
3. APK is signed with the release certificate(s). (Important: with "App Signing by Google Play" it only works if you download directly from GooglePlayStore!)
4. APK is uploaded to alpha/beta distribution channel (previously - as a draft) to the developer console at least once. (takes some time ~2h-24h).
5. IAB products are published and their status set to active.
6. Test account(s) is added in developer console.

Testing requirements:

1. Test APK has the same versionCode as the one uploaded to developer console.
2. Test APK is signed with the same certificate(s) as the one uploaded to dev.console.
3. Test account (not developer) - is the main account on the device. (Main account might be not necessary - according to @MinosL comment)
4. Test account is opted-in as a tester and it's linked to a valid payment method. (@Kyone)

P.S: Debugging with release certificate: https://stackoverflow.com/a/15754187/1321401 (Thnx @dipp for the link)

P.P.S: Wanted to make this list for a long time already.

Thnx @zlgdev, @Kyone, @MinosL for updates