Art, like any form of portable property, is easy to barter and sell without paperwork. The fact that it is easily transferred without documentation makes it a perfect medium for money laundering. Another reason it is an appealing way to launder money is because it doesn't have a mainstreamed mode of dealing. You can just as easily "purchase" from private sellers as from reputable auction houses.
Art is the perfect medium for crime. It offers exchangable commodities that are really only worth what a person claims they are worth. A cash payment of $100,000? No problem! You got this painting! And isn't it exquisite? Worth every penny.. If someone disagrees, you could simply tell them that their sensibilities follow a different school than your own.