But with art there is only one work produced by an artist, or if a series, probably some way to properly id it based on a number of factors.

With say a sweatshirt produced by a third party based on some digital stencil art, do you know for sure the one you're receiving was printed by the company and given to an employee/customer? Literally hundreds, possibly thousands would be in circulation. How would you be able to distinguish that from say something a former employee w/ access to the original art data having it reprinted?