We're launching an app that is selling the service through inapp purchases. Currently we asessing a risk of Jailbroken devices.
Is it possible that a user with a jailbroken iphone simulates an inapp purchase without paying for it?
yes, it is possible that some app can simulates an inapp purchase without paying for it.
To prevent this, you should check the IAP transaction and receipt from app store. You can reference Verifying Store Receipts from the Apple In-App Purchase Programming Guide.