I don't think you can buy Gift Certificates with the credit.
The books are registered to the account they're bought on, and can only be used on devices that are registered to that account.
Devices can only be registered to a single account at a time, and must be connected to the PC to do the registration. Also you lose the books from an account if you de-register your Reader from that account.
About the only way I can think of is if someone else let you log into their account, register your Reader to it, and then de-register it.
I suppose if they hadn't added any Credit Card info yet, and changed the password on the account after you went in and did your register/deregister, somebody might go for it.
