I believe the add-ons will always show on your purchase list but will be licensed to the site. Site ownership can be transferred pretty easily.
I've done some blogs covering how at http://blog.arvixe.com/adding-contibutors-to-a-concrete5-project/..... and http://blog.arvixe.com/transfering-concrete5-project-ownership/......
Once the site ownership is transferred, only the new owner can release assigned licenses (effectively making your customer the owner of the add-ons as he/she will have full control of their use)

I hope this helps you out

Adrian
Arvixe Web Hosting / Concrete5 Community Liaison |
http://www.arvixe.com/concrete5_hosting...................

Thanks Adrian for informative reply...It's really helpful.

Its really helpful.