Well you could buy a second level domain like: domain.com and sell subdomains of this. I can't think of anyone who would actually buy those, you may need to host the Authority for domain.com as well, so further investment.
You will also need a static Routable IP address to point to your Domian Name, and anyone who buys domains from you, and of course the DNS server that will be the Authority.
Look put it this way, the barriers to entry of this market are huge, the costs are significant, its very technical and specialised, and there is hardly an exit strategy. There is a very good reason why only Telecom companies really do this, and not posters on BB Boards