I already have Metronic 8, I purchased one copy for my client, and I already used that licence.
Now I want to use my already developed application for another client. So now how I will purchase a licence and activate my already developed application.
Or if I buy another copy from https://themeforest.net/ then may I able to use this new licence for my old copy that I already developed for my new client.
You can reuse your already developed application for other clients as well by just purchasing a separate license for each new client. You can reuse the same code for any number of clients and deployments as long as you have a separate license for each client.