Buying a car from a guy you know or highly suspect used the car to rob a bank and is selling it to you for the express purpose of hiding what car was used in the bank robbery sounds like a crime (or at least something that would land you in hot water).

Buying a gun from a guy that you know or highly suspect used the gun to kill a guy and is selling you the gun for the express purpose of hiding what gun was used in the murder sounds the same.

^Those are closer to what money laundering is.