I was already asking about this feature, but it seems it will require custom logic as it's not implemented in the original forum software:

 

TL;DR the plugin should just do:

A key costs 2.99$ which works for only 2 days.

Once it's purchased from a reseller, the actual expiry date to be at least 6 months for that same key.
And only when the reseller finds and sells that key to his customer and the customer activates it for the first time to modify the expiry period from 6 months to 2 days.
So the customer starts the actual "timer" of 2 days once he activates it once.

@DawPi Hello mate, I am tagging you because you might be interested.
 

PM me then. 😉