Your question is too vague to be generally answered. If you have a specific circumstance mention it and you might get some answers. In the meantime operate as if the customer is always right.
Before the 'marketing people' can manage the customers, you have to get the customer. You get the customer by convincing them that you can help them achieve the result they desire. Things that could help are the ability to communicate with the customer, proper grammar and spelling, and being able to help the customer realize exactly what they want - most of the customer doesn't really know what it is exactly that they want, but they know what they want to achieve (eg sales, referrals, etc.)
Sooner or latar you may have customer request for refund even they have your digital product or you have stated clearly in your sales letter there is no refund. In such cases, i will refund them as some may even go to forum and complaints if you refuse to do so. Keep your cool when you encounter any unreasonable customer request.
Hello Dear, First of all keep communication channel open and easy for your customers. Listen to their feedback. Find out how they feel about your product/service. Encourage them to comment how to improve services. Document all relevant points and take action quicker than your close competitor. InternetAd.biz