If you are an admin you can do the following steps:
1. Make one fake customer.
2. Set the site into Maintenance Mode, be sure you keep on logging to the admin
3. Now activate the Cash on the Delivery method of payment.
4. Complete the buying process with many quantities with the use of the Cash on the delivery payment method
In this way, we can show the products into the best sellers.
If you are looking to show or add the bestseller module in pages then this video will help you: