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)

If bitcoin is not treated as legal currency, then the person you bought the car from would be required to return half the coins you have them to the original owner as they would qualify as stolen property.

This risk would have to be managed by contracts, insurance, and/or quick exchange for safer assets.

The point isn't to make the coins themselves clean, it's to exchange them for something clean.

So a laundering service could specialize in buying tainted coins and to sell them off to other unsuspecting people, pocketing the difference. In return they could provide clean coins for their customers.