Definition: The information you have on your customers, including but not
limited to their contact information, how often they purchase from
you, what they purchase and how they pay their bills
Your company's sales records are quite likely to prove your most valuable marketing information source. Files of customer purchases with addresses, amounts, dates, products, payment methods, returns and other information constitute a rich trove of marketing data. You can analyze this information to find out who your best customers are, what they like the most, and what kinds of marketing approaches appeal to them most powerfully.
One of the best things about doing market research with your own records is that it's all proprietary. No one else has the same data you do, and there's no information anywhere that is more appropriate to your business. Add in the fact that market research using your sales records is economical, and it's a powerful combination. Do everything you can to capture and analyze information from your own sales.
See also "Customer Relationships."