How to link paid app user account to the system ?

[AndroidBeginner]

Hi guys,

I have an issue related publishing the paid app to android market. (My application is internet connection based app.)

If I've put the app to the android market, can user who bought the app pass to anyone ?

How is its security (I mean safe of .apk file) ?

Also, what is payment tool of android market ?

My main point is choosing the best way to link paid user to our system. Actually I don't know how to link paid user account to my system(by email address or device unique id ?... what is better way ?).

Can you suggest me on this part ?

I really appreciate for help.

Thanks in advance.