As with virtually every industry there are generalists and specialists. People who work in some industries have knowledge about fringe or related businesses, but their primary job may not be what you need for your project. As an example, if you have a plumbing problem you wouldn't call an electritian to do the work. Even though he may have knowledge of plumbing, his training and experience are definitely NOT in the area that you need.
List Brokers fall into the same classification. If you are looking for a mailing list why go to a printer, ad agency, or search the web? Although these may give you what you are looking for, are they specifically trained to make sure you are getting EXACTLY what you want in a format that you need it in? When was the list last cleaned or updated? Does it include marginal addresses that may not be deliverable according to the United States Postal Service? How is the list compiled? Who is the contact name ( if any) on the mailing list? Also, you have to keep in mind that printers and agencies tend to mark up their lists. So you are paying more than retail for your list. Something that will never (or should never) happen when dealing with a list broker.
All of these questions and hundreds more are what List Brokers do for their clients. Many people assume that they are saving money by letting their printer or ad agency purchase their list, but list brokers work in a unique environment where they are paid by the list owner to order their list for clients. The amount you pay is exactly what it would cost if you went directly to the list owner BUT the List Broker will do all of the leg work for you. They will prequalify the list, find out when it was updated last, if it is CASS Certified which will allow you to get the lowest postage discount possible, and if the list can be segmented by the demographics you need for your marketing efforts.
All of this work is done for you at NO extra charge. I recently did a search for a client and came up with six list recommendations for their direct mail project. My client only needed to evaluate each list, the source of the information, when it was last updated and the price. From there, they selected the lists that best met their needs.
In addition, List Brokers can help you understand that it's not just the list your buying, but if you are purchasing multiple lists, merge/purge them against each other, and the multi hits (names that fall into more than one list) are ideal for future mailings. Afterall, you paid for the name, use it!
As a List Broker of over 20 years I value the relationship with my clients. Finding more information and better lists for my clients makes them more effective AND brings them back for more. Don't look at your broker as a vendor, but rather a partner. Someone who is as interested in your success as you are. And, someone who can save you money by doing all of your research for free and NOT marking up your list costs.