Hello, @ami. Please accept my apologies for such a late reply. 
That’s correct! It is possible to set the platform commission on the admin page only:
Under the manual template customization yes, it is possible to add such a feature. But the original template functionality does not support such an option.
I suppose you might need to create an additional data type (column) in the Users table that will be responsible for the user’s commission percent and should be applicable only for the users under the “seller” type. Then, you can use this parameter in the Charge user action (field “App fee”).
It is possible to add this feature under the manual customization also.
No, I’m afraid. There are no tutorials or documentation that might teach you how to customize the Marketo template. The current template’s documentation is representing the list of available functionalities, structure, and some points to note about the template.
For comfortable template customization, we strongly recommend obtaining at least solid Beginner skills. These resources should be useful:
Let me know if you have any more questions. I will be glad to help. 
Take care.