How It Works
You enable a client as a reseller in the Resellers tab and assign them to a pricing group.
The reseller opens the reseller area in your client area, copies their API Endpoint and API Key, and downloads the registrar module ZIP.
They install that registrar module in their own WHMCS and paste in the two values.
Their domain orders are now forwarded to your WHMCS over the API, charged against their credit balance, and fulfilled by whichever registrar you have configured.