A list of prospective customers, victims, etc: The directory is not intended to be a sucker list(1910+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with sucker-list
A list of names of likely prospects for making purchases or donations, as in Some charities raise money by selling their sucker lists to other organizations. This term uses sucker in the sense of “dupe,” a usage that in turn alludes to the naiveté of a baby suckling at its mother's breast.
[ ; 1940s