When I sell a website based on a themeforest theme, do the customer has the right to have my purchased license for free ?
No they have access to the project that you used the item to create.
They will never have direct access to item support or updates.
For the same reason, you should create a new account for each client and give the login details to them to get the updates in the future. It’s usually problem.
No, your customer need to purchase the item separately.