What application fee should we charge?

We are building a network where anyone can publish and (coming soon) sell guides. We about a week away from launching the ability for users to create their store. In order to sustain the application and business we will need to charge a small percentage during a transaction. We plan on being transparent about it, but am looking for opinions on what percentage would be appropriate to charge?

…3%, 5%, 9%, 12%?

Any advice?

Project: https://www.yadayadayada.io/