地址：
https://developer.apple.com/library/ios/technotes/tn2413/_index.html
最容易被大家忽略的一点：
What url should I use to verify my receipt?
Use the sandbox URL https://sandbox.itunes.apple.com/verifyReceipt while testing your application in the sandbox and while your application is in review.
Use the production URL https://buy.itunes.apple.com/verifyReceipt once your application is live in the App Store.
Note: Be sure to send your app‘s receipt for validation to the App Store.
Always verify your receipt first with the production URL; proceed to verify with the sandbox URL if you receive a 21007 status code. Following this approach ensures that you do not have to switch between URLs while your application is being tested or reviewed in the sandbox or is live in the App Store.
The 21007 status code indicates that this receipt is a sandbox receipt, but it was sent to the production service for verification. A status of 0 indicates that the receipt was properly verified.
原文：http://my.oschina.net/u/1024573/blog/518886