Now you're getting smart. Now you see the value of specifying and purchasing the correct mailing list. You can see your mailing go from no response to profitable in a hurry, just in the extra attention and correct selection of your mailing list. Ask brokers for a data card, which shows list specifications. When purchasing names to sell products to, ask how recently the people on the list have made a purchase.
Recency is a key factor in mailing lists.
The formula recency, frequency, monetary is the standard for measuring the quality of most mail order purchaser lists. Ask how often the list has been rented. You don't want to get a list that has been rented too often and may suffer from list fatigue. Or a list that is never rented, because there's probably a reason no one is using it - no one is getting a response from that particular market or that list. Ask how many other mailers have tested the list.