An unhappy customer deserves to be heard, and understood. There is normally a basis for their distress. It is a good business practice to salvage the good will of each customer, even if it takes some extra effort to do so.
The first step towards handling this situation is to listen, not talk. Most customers want to have their say, and interruptions to their explanation as to why they are unhappy will only anger them. Listening will also make you understand the source of their anger.
Once the customer has covered his points, I suggest that you agree with them as to why they are unhappy, even if you do not. This will immediately make them happy with you, at least for the moment. The task at hand is to come up with a solution, or compromise which will be acceptable to them, and your business.
Should their displeasure be something for which there is no solution, such as buyer's remorse, and store policy will not permit a return of the product, tell them the truth, not a bunch of bull. Never permit yourself to be brought into the middle of the problem, because then it becomes personal.
Offering a course of action other than arguing with you is the best bet. Most larger companies have a home office with customer service telephone numbers, assist your unhappy person by providing them the means to continue their battle, if they wish. The whole point to this process is to salvage this customer for the future it that is possible.
It always pays to hear out the complaints voiced about your company. The lessons learned can help you improve in areas that cause dissatisfaction, remember, the customer is always right. Additionally, that is often true!