Make sure you ask tons of questions before handing over any money. So make sure you get tough with them about answering your questions. If a list is clean, you won't get a lot of your mail pieces back. 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 many other mailers have tested the list. Ask how many people continued after their test - meaning their test mailing drew a response and they mailed to it again. Then ask how many people rolled out - mailed to the rest of the entire list. Ask the origin of the list.