How to handle User Accounts?

What is the right way with Accounts on Envato?
I buy and use a Wordpress Theme for a client with my Envato account.
When work is done and the Website is finished I transfer the theme-license to the client. In the Wordpress Backend I put my Username and my API Key from Envato.
For the next client I go th same way. So in each Backend of each client I use the same Username and API Key.

Or should I open an new Envato account for every client?

I suggest you can ask client to buy that theme for you.