We are working on an iOS application that provides some credits (paid) that can be used within the app. We have few queries regarding this -
Please go through apple review guidelines and your doubts will be cleared.
You can use third party payment gateway,if you are offering physical goods,service outside app. If you are selling your content or service inside app then you must use inapp-purchase.
As per purchasing currencies guideline,
Regrding your questions, answers are as below
You may read Apple review guidelines for more information.