Ask how many ordered names for another continuation, meaning they absolutely did make money and it was worth the effort. Then ask how many people rolled out mailed to the rest of the entire list. Ask the origin of the list. A common source of names is records that have been compiled in some fashion. Another common source of names is magazine subscribers.
Trade associations are usually an excellent source of mailing lists.
Better associations always contain the major industry players. Local associations like the Chamber of Commerce are good for local business names. Two excellent resources for investigating lists at the library are the SRDS Book of Lists and the Ox bridge Communications National Directory of Mailing Lists. We use the Oxbridge Directory in our own office and it's thorough and exceptionally easy to use. List brokers are found in the phone book in every major city.