Thank you LePoulpe,
What you store in the database is entirely up to you. If you want to make a IAP system or an online Highscore system.
The extension relies on the Javascript SDK for Flash and all others (cpp = windows, mac, iOS and Android) on the REST API.
The quick glance I gave it didn't mention a REST API method so the information in the blogs you posted is not directly possible yet.
Getting the money and all that flow is not easy when you are outside of the Apple Store and I Guess the Google Play environment. Parse will not handle that stuff, that should be done by you or by the store!
All communications is done using HTTPS (=secure socket layer) so you can pretty rely on it being safe.
Most important thing is not to give away your Parse KEYS in documentation, exported .Stencyl or any form of communications. Better don't give anybody the sources or access to your computer at all

Hope this helps. I don't have experience with in-App Purchases myself so maybe somebody else can give you tips about it.
Best regards