Themeforest themes license?!

I wanna ask about the license of themeforest themes, is it allowed to use the same theme (demos) available on it for many websites and sell the websites for varies customers?! Or use the same demo from the theme for multi domains?!

Another Question: How to find a solution for stealing the theme from other developers I deal with? or from any one I sell the website to? how to protect my license?!

I read this page :

You have to buy a new copy/license for each individual site you build

There are various different ways in which authors protect their items and unethical use including API or purchase code recognition.

If you plan to use files from here for client work then you would both be much much better off to have the client purchase the item and not you.