DNS Naming Convention

[Doug]

Let's say I have a domain on a shared host (www.host.com) called
www.blahblah.com

At the registrar, I listed ns1.host.com as the DNS.

Then on a new server I set up my own name server and call it
ns1.blahblah.com

Now, I go to the registrar and point a different host
www.blahblah2.com to use the DNS of new server, ns1.blahblah.com
It then prompts me to enter ip address of ns1.blahblah.com which I do,
and after 24 hours am able to visit www.blahblah2.com on the new
server.

My question is, what's to prevent me from naming my name server
ns1.yahoo123.com, as long as it's not registered in any DNS servers,
should I be able to use it? And what would happen if yahoo decides to
buy the yahoo123.com domain and wants to use the ns1.yahoo123.com
domain as their own name server?