So, it is slightly more complicated than a serial number. In some sense, everytime you make a transaction, you burn all the input coins and creates new coins for the same value, each assign to the respective recepient. Tracking them is not straight forward. Most OTC are not very tech savvy and I can imagine one either being criminal but stupid for bringing it to Bitfinex or simply non criminal but did not do their due diligence and KYC the seller (which is very surprising knowing the space)