“The job tracking down abstract expressionists is an unintended consequence of a little-known 1977 [California] law designed to cut artists in on the profits from the resale of their works. … In theory, the law is a boon for artists. In practice, it means [Patty] Milich sometimes must spend months trying to deliver paltry sums to people who have faded into obscurity, moved abroad or simply don’t want to be bothered.”