I feel that taking unstamped stamps and reusing them is un-ethical and may even be illegal. However if I get envelopes with unmarked Australian stamps that have not been destroyed by black marker pen I take them into my post office who are very obliging and they stamp them for me. So I regard these as used CTO.
See below cover I received yesterday with a great array of unmarked stamps, and a miniature sheet which was CTO back in 1980 and applied to the envelope. So they are now used and could be regarded as one or the other catalogue listings of Mint or Used.