A sucker list is a list of people who have previously been successfully solicited for something.
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.
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 is fraudulent racetrack touts, who, in particular, used to call people known to be in deep debt and give them false tips.
See also 
- Q&A: how to avoid becoming a sucker - Times Online
- "Sucker List", an article from the November 18, 1929 issue of Time Magazine
- Reloading Scams: Double Trouble for Consumers