Dear Friend,
It is a known fact and the same is already expressed by some of our learned friends.
There are 3 options provided for in the Rule 3(5) of CCR,2004 and they are briefly,
Clearing as such by debiting the amount of credit availed ; clearing after usage of some time by deducting 2.5% of credit for usage of each quarter and pay duty on the transaction value if the CG are sold in form of scrap.
Since, your subject query is what is the duty to be paid when you are selling/clearing the used CG by way of scrap, The answer is you need to pay duty on the transaction value at which such scrapped CG is being cleared/sold.
Best Regards
Suryanarayana