It certainly is the case that in years gone by 21 keys at T keys were part of the necessary kit of any self respecting mailbag thief. So such keys HAVE most definitely been used for illegal purposes. However, another question arises, BR keys, and those issued in the since the demise of BR, remain(ed) the property of the issuing authority and should have been returned upon retirement, or other end of employment.
Despite a comment to the contrary, there is no specific criminal offence of wrongly possessing security issue items.
So, does that mean the BTP would be interested in the appearance of a T key on EBay? Somehow I doubt it.