Selling my own subdomains as regular domains

[axho]

I know that I can add own subdomain for client who want start hosting on it.

But I want to sell domains (without hosting). Besides regular domains (.com, .net, .org) I would like to sell subdomains of my own "functional" domains e.g. .4x4tech.pro
Clients should have possibility to search within database and add to basket any available domain with suffix .4x4tech.pro for example: johnsmith.4x4tech.pro and js.4x4tech.pro and hobby.4x4tech.pro
System  should have got functionality of a registry of such domain and keep a database (registry) with all registered domains with such suffix.
I should have possibility to set up a price for such domains.

Is there possibility to use WHMCS for such business?

[RadWebHosting]

I've never heard of registry running on WHMCS, but that doesn't mean it hasn't been done. You probably want to start with something like Nomulus for the registry services. 

You could definitely create a registrar API for interacting with the registry service. In fact, WHMCS provides sample code for creating a registrar module on Github: https://github.com/WHMCS/sample-registrar-module

[lulzkiller]

Best bet is to contact modulesgarden, and ask them to code this feature for you in whmcs, you will not find it out of the box.