Wikipedia
A sucker list is a list of people who have previously been successfully solicited for something.
The major areas of sucker lists are solicitation of donations and fraud.
An early example of sucker lists made public is given in the November 18, 1929 issue of Time Magazine in an article about the United States Senate probing into one lobbyist.
People who become victims of, for example, a telemarketing fraud, often are placed on a sucker list. Sucker lists, which include names, addresses, phone numbers, and other information, are created, bought, and sold by some fraudulent telemarketers. They are considered invaluable because dishonest promoters know that consumers who have been tricked once are likely to be tricked again via the " reloading". As a result, these persons became flooded with letters, e-mails and phone calls with various lottery wins, investment plans, get-rich-quick schemes and work from home offers.
The consequences for the victims may be devastating, ranging from further loss of their money and/or assets, and at worst, being forced to change their identity and even commit suicide.
Yet another usage was described in the movie Sucker List, a part of the 1941 United States series Crime Does Not Pay. The subject of the movie are fraudulent racetrack touts, who, in particular, used to call people known to be in deep debt and give them false tips.