Using premium plugins without purchasing

Hi Guyz ,

I have a little query .
I heard from my seniors / friends / some companies and other people around web that you can not sell premium plugin , If you If you can't provide support
One reference is -

" you cannot charge for code, but you can charge for distribution."

Is it correct ? And If I will create a paid plugin and sell from my website , than how can I find that who is using my plugin without purchasing from me , And how can I track that if user purchase for 1 site then using on how many sites ? May be If other user can modify my plugin and sell with his name ; how can I stop him and what legal actions can I do if someone is using my paid plugin illegally ?