Protect my EA with License Number

Hello everybody,

I have an EA available for selling to my customers.

I would like to protect my EA with a license number that can allow ONLY the people that will buy the EA. The license key I want will be available only for one account each person, or something that I will put into my EA the account number of my customers that will buy it.

So in this case I don't get the risk that people will pass it each other.

Someone can help me? 

Please apply explaining how you will proceed and how it works.

Thanks a lot.