Tempting, but not for me my friend.
Taken from WHMCS Site: "Support and Updates was part of a legacy "Owned" license type which was removed from sale in 2016 and licenses grandfathered.
As of 6th April 2021, Support and Updates have been discontinued and are no longer available. It determines eligibility to receive technical support and software updates. The following does not apply to monthly leased licenses."
Would mean I couldn't update to newer versions, although I would get security updates. I'll be moving away from WHMCS myself also next year I think as even their monthly paid subscriptions are getting stupid as they are hiking prices AGAIN in January.
I wish you luck with selling it though, It has to be useful for somebody