How are sellers paid at photo stocks?

Question

Mobile development

There are quite a few mobile apps on Google Play and Apple Store that allow contributors to sell stock photos. I have a few questions from the creator of such an application:
1) Before allowing users to sell photos in the application, are they required to pass verification and sign a contract. Or is it possible to initially allow selling, but withdraw money only after verification and conclusion of an agreement?
2) When selling photos, is the money of the sellers stored in the Google Play system and the Apple store, or is it with the owner of the application?