In the past few years I've owned four Kindles. Three of those four I de-registered online and either sold or gifted to someone else. This is a normal practice and I've had zero problems. Amazon could care less as long as you properly deregister it before trying to register it with another account. I even forgot to deregister my first Kindle when I sold it, but a call to Amazon CS resolved that issue quickly and satisfactorily.

Bottomline is that Amazon will happily sell you stuff through any legitimate account. They only become concerned if they think your Kindle might have been stolen, and that is a good policy IMO.