In Magento Marketplace module, admin can create the customers and sellers both.This can be done by creating the customers first from admin panel and then approving that customer as a seller.
For this admin needs to navigate to Customer>Manage Customer.
From here admin can either edit a customer or add a new customer.In order to add a new customer, the admin needs to click on Add New Customer Button whereas if he/she wishes to edit the customer he/she can click on Edit button.
On this page, the admin can navigate to ‘Do You Want To Make This Customer As Seller?’
Here, the admin will enter the shop name of the seller and select ‘Approve Seller’, to make the customer a seller.