the item you requested is not available for purchase

Here are some things to check:

1. You've created an apk and you've published it to the GooglePlay Dashboard in Alpha or Beta.
2. The app in the GooglePlay Dashboard is NOT in Draft mode, but in Published (you'll need to make all the small circles with the check icon in them on the left side of the screen green before being able to publish).
3. You've set another test account than the one that's "attached" to the GooglePlay Dashboard. You can do that by creating a Google+ group, add your test account to that group and specify the Google+ group in the GooglePlay Dashboard.
4. The apk that you're using to test the purchase has the same version code, version name, and most importantly it's signed with the same keystore as the apk that you've published in the store.
5. You wait a couple of hours between when you change something in the dashboard in order for the changes to propagate. It takes a couple of hours to do so.
6. Make sure the sku value is a valid sku value (compare it with the one you've entered in the GP Dashboard).
7. You try to purchase an already purchased item. Get the purchased items and display them in the log to see if so. If so, then consume that product or refund the money to your test account(you'll need to wait for the refund to propagate. It takes a couple of hours.)
8. Make sure the Inapps are Active !

What did it for me, after 6 hours, was this last part:

9. Make sure you're signedIn into google (in your browser) with the test account and you open this link (marked with the red) and you approve to become a tester !!!! http://i.stack.imgur.com/VSgAV.png

UPDATE

10. If you're using flavors to build apks targeting different CPUs and you build for arm, armv7 and x86 and you upload them all to the store, remember to use one of them to test the inapps. If you use a "universal" build which contains all the cpu libraries (basically another build than what's on the store) it won't work.

My experience on this error is:

1. Make sure to upload the signed APK to developer console.

2. Make sure to install the signed APK on your device not launch the app in the debugger.

3. Make sure to create a test account in your developer console.

4. Setup you testing account

   1. Make sure to sign in your device with your test account.
   2. In a case of closed alpha/beta testing, make sure you have added your test account to selected testers group, you can do this on the page of management your alpha/beta version.
   3. In a case of closed alpha/beta testing, make sure your testing account have accepted participation in testing of this application via special invite link

5. Make sure to create in app billing in your developer console and finally activate the item from the console!!! (this is the one that got me after fully following google's tutorial)

6. Make sure to set VersionCode and VersionName in the manifest to be the same as the version in the developer console (Alpha, Beta or Production. Drafts does not work anymore). @alexgophermix answer worked for me.

If you're looking for testing payments without actual money withdrawal, you need additionally add your testing account to another special list of accounts "Gmail accounts with testing access", you can find that on play account setting(not project!). On the page where you can see a list of your projects go to settings, and then on "Account details" page (scroll down, it's almost in the bottom of page) you will find the "Gmail accounts with testing access" list.