Q:

How to link paid app user account to the system?

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.

A:

the Android Market use Google Checkout for payment. You might want to check if you live in a country currently supported to sale Android application (I've learned the hard way...). 

Austria
France 
Germany
Italy
Japan
Netherlands
Spain
United Kingdom   
United States
etc.

Edit: The list is now much bigger than that. The below link list them all.
https://checkout.google.com/support/sell/bin/answer.py?hlrm=fr&answer=150324
There seems to have some issues with the apk protection (look for android .apk copy on google). As far as I know, user can't pass the app to anyone (except if they crack the drm of course). Average user probably will not do this.

